I have to disagree almost completely here with you. Adding mechanics does add a new skill level, you are correct here. I might use sprint not always to my advantage. I know the game pretty well, not to sound like a prick or something, but I would consider myself way better than the average player.
Now to your second part. Game advancement is not always what keeps you playing, I've got a few examples here for you. Not all the games will get less population when they get older. Halo 3 is not played that less because it's 8 years old, but because there are 2-3 new halo's since halo 3's release in 2007. Of course people will advance to the newer title. That's why the xbox 360 is getting played less, there is a new version. And who said copy pasting a game would defeat the purpose. This might sound a bit cliché, but CoD in the eyes of almost all the people who play it, is the almost the same game every year, with new maps and new graphics.
And then there is Counter strike to talk about. That game is only growing and growing right now. Every month a new top player count is reached nowadays for cs:go (released in 2012). But the game still has the same maps as counter strike 1.6 which was released in 1999. Over 15 years ago! Mechanics have not changed that much, no big gametypes have been added, the principle of every game is the same but still. Counter strike: global offensive is in the top 3 of most played games on steam. Because it is the latest installment. And I don't expect Valve to release a new CS before 2020. The playerbase will not degrade much over that time, because there is no new installment. Only new graphics and a slightly different map layout. Hell, it does not even have a single player to play for.
If halo 5 would be the same as halo 3, I would have been dissapointed, sure! But there are things I like more in halo 3 than halo 5, and vice versa.