I'll like to clarify a few things since you are evidently not informed with Halo since Halo 3 started to drop out of the top 5 most played games on XBL.
Jet Packs and armour lock have not even been put into any form of Halo 5 Guardians, on top of the Battle Rifle was in the beta. Overshields are back as a map pick up being a one time use equipment. Halo also didn't die after Halo Reach, it died after Halo 4. While Halo 4 was a financial success, it failed with it's population and this can be pinpointed to two main factors.
Factor One; Black Ops was released within two weeks of Halo 4's launch. Due to the popularity that Call of Duty had at the time (it was the most played game on XBL in 2012), it can be argued that Black Ops ruined any chance of Halo 4 being confirmed as a top 5 game for XBL. However the second factor is more likely. Factor Two; Halo 4 was too different for the average Halo fan by comparison to the original series. The introduction of mainstream FPS elements were undesirable by the community and saw the rapid then steady decline of concurrent players for Halo 4, see below in spoiler tag.
We can then look at Halo Reach, while Halo Reach was not as successful in its population as Halo 3 was, it was very close to being the same after at least 1 year from release for both games.See below the population spread over the month of June for Halo 3 in 2008 (one year after launch) and Halo Reach during 2012 (two years after launch).
Furthermore, an analysis for Halo Reach's population count over a period of 12 months between January 2011 and January 2012 can be seen also. This ties into the next point I'm going to make about the population of XBL games in general at the time.
After the dynasty of Halo 3, other games became more prominent on XBL's service. Xbox 360 was just a bundle of exclusives, but instead had multiplatform titles from big AAA publishers being shoved onto the console en mass. Call of Duty was the most solid game for years, peaking at the number 1 rank for so long, even when Halo 3 was one of the most player games at the time. Whenever Halo 3 was down at rank 2, it was because Call of Duty was rank 1. As seen below, there is a good comparison of every Halo games ranking, no other games are included for the sake of simplicity.
We can then introduce how Halo 5 Guardians' beta went. During the 3 weeks that the beta was live, the game still managed to poll over 2.5 million hours of gameplay from the participants. A full detailed image of statistics for the beta is seen here;
It's evident from this that 343i did something right, especially given the dramatic failure that was Halo the MasterChief Collection. Halo MCC failed dismally and still does not work on all regions correctly, and even in USA where the main servers are located, the game still doesn't run flawlessly. Granted 343i were given horrid servers to run the game on, the coding in the game itself was subpar and 343i deserved every bit of aggression they had directed towards them. That being said, the turn around that was Halo 5 Guardians' beta was remarkable. The netcoding saw you be paired with players and had a game started up within 30 seconds regardless of region. The exception was when players weren't being paired with any games at all, which was only fixed late towards the beta's cycle, although the population for that error was incredibly minor.
Moving towards your main topic about the 1-50 Ranks, it can be safely said that the removable of that specific rank system did infact not kill Halo, it was the market for games that did. However what makes the ranking system good in the first place? Lets look at the first argument you posed, which was about map control. Halo has pride itself upon map and power weapon controlling. Even a game as iffy being Halo 4 can proved that game control is paramount. Halo 4 did have the ordnance system, however if the opposition team was communicating well enough and were decent, the weapons the opponents got should mean aboslutely nothing. You should always be able to teamshot power weapon holding opponents with a standard loadout. If you die, that's your own fault for not working at a maximum efficiency. This can be proven in MLG tournaments, pros who have power weapons do not go crazy about them, they still work with the team. It's the oppositions task to eliminate that threat, and most of the time, they do. So again, is this why Halo 2 and 3 did so well with their population? No, because Halo Reach, Halo 4 and to a lesser extent so did Halo Wars. IT was the market that took over from Halo's reigns.
What about weapon and equipment spawns? The argument can be said that without these consistent spawns, the game would no longer feel like Halo. While that is a popular argument, due to Halo 4 worked with the ordnance system, it's not a correctly justified argument. Spawn timers only happened after the weapon, vehicle or equipment was used up. For example, if I picked up a sniper at time 08:00 and I died at time 02:00 the sniperwould not spawn again, Halo 2 and Halo 3 used this flawed system, Halo 4 fixed it. A game that works around this, which is closely tied to the Halo series is Destiny. Destiny uses static spawns, meaning everything that does spawn, will spawn at specific times, regardless of when the subject in question was used or not. This is a superior spawn system, and Halo 5 Guardians does infact use this. 10 seconds before the weapons spawn into the map, a comm speaker comes online for all players (this is an audio option that can be turned off in the full game) alerting them to the static spawn of a single weapon in Arena (the competitive gamemode for Halo 5 Guardians). It's also important to note from Halo 2 and 3, that if you stood for 3 seconds with said weapon or equipment, the timer would start up again. However only you and you communicating team would know this, there would be absolutely no way for the opposition to figure out when the timer is going to restart again, giving the controlling team complete and utter advantage, an unfair advantage since it is not based on skill in the slightest.
You also make the argument of the Battle Rifle being "Halo". I then pose this, what makes "Halo"? Because if I recall correctly, the Battle Rifle did not make an appearance in Halo Combat Evolved whatsoever, and that is the most pure form of "Halo" there is. Halo is easily defined by three 'tools' in a combination; weapon/grenade/melee. In Halo 2 and 3 the Battle Rifle was the only main weapon worth using, the other weapons were bullet hoses which did absolutely nothing, further showed off by MLG players only going to pick up a Battle Rifle and refusing to play non BR start games. The Battle Rifle was literally a cancer that ruined fair gameplay, Halo 2 had the highest bullet magnetism in the franchise (see
, enabling easy cross-mapping with the Battle Rifle without even trying, it was not a weapon of skill. Halo 3 also had the issue where the main gun, the Assault Rifle did next to nothing and the Battle Rifle was sought after at all costs in most standard Ranked games. Halo 5 Guardians fixed this making the Assault Rifle a great weapon to use in close-medium ranges, and limiting the Battle Rifle to only mid range dominance while lowering its reserve capacity. Halo 5 Guardians also brought in the DMR as a good long range counter for the Battle Rifle, the Battle Rifle will now most likely lose every encounter at long range, so long the opposition is using a DMR. However 343i also countered the DMR by making it incredibly difficult to use at close quarters on top of that it also has a very slow time-to-kill in comparison to close range weapons. So was the exclusion of the Battle Rifle in Halo Reach what caused Halo to die? I beg to differ.
Now we can finally move onto the actual topic of 1-50 ranks. Were they good? Apparently a lot of people liked them, but was it blind and bias love? I say yes. The 1-50 ranking system was fundamentally flawed as it did not represent any quantifiable numerical solution to your skill. It only showed how many times you won without massive losing streaks, without taking into account how good you were, or how badly carried you were. A very small population could carry themselves up to rank 50 without a team, and those are the very very few people who could say that had legitimate skill to get to rank 50. Those players are primarily pro players, not the average bloke. Additionally, Halo 3 was infamous of businesses starting up for carrying people to rank 50, or even in many cases, taking over accounts to get them to 50 for a price. These sorts of businesses were evident all over Halo 2 and 3 as you could curb stomp so many players if you were a level 5, just due to a vast amount of the population getting other people to boost them up. Game boosting was also another issue, being in a party with the right amount of players who would 'just happen to be partied up' in the game itself then would purposefully lose so one player could get to rank 50 in a certain playlist. You may say that it hardly ever happened, but it was a very vast majority who did, this was seen when Halo Reach came around the corner and people couldn't hide behind a rank because it meant nothing. The ranking system also failed in the regard that if you quit out or were dropped from connection from any means (DDOS'd, ISP dropping, etc) you would go down rank. No matter how legitimate the reason you had, you would go down rank. This meant that even if you were at 50, you were always posed to the risk of being dropped in rank without any good reason, only for you to put so many more winning games into your account to go back up 1 rank. Is this a good ranking system? I think not.
Halo 5 Guardians did improve on the ranking system, it is based on wins, and how your team wins. If you do well on the losing side, you aren't penalised as hard because it takes some form of individual analysis to determine what sort of rank rearrangement is required. If you do exceedingly well on the winning team, you will also get a slight boost to your rank up in addition to your winnings that the team overall is rewarded with. However the system is not perfect, if your team is curb stomped yet you were the only great player on it, you will not be saved from ranking down. Same to if you carried your team so hard for a win, you do not get a massive boost in rank, because the system is based on team winning, not individual scoring (a decision I disagree with). However I can say though, Halo 5 Guardians has a much better ranking system than any other Halo game has, no other game comes remotely close. So in conclusion to your claim, the 1-50 ranking system had absolutely nothing to do with Halo's success. The fresh game being out in the market with no competition on the Xbox console is what made the game a financial and population based success. As more and more games started coming to the console, more and more people left.