It's hard to see Halo ever be a thing without splitscreen, but even so you should've researched the game before buying an Xbox solely for it. That's ALLOT of money to spill down the drain, so an investment of that sort should've been an informed one. This is exactly what Publishers want the consumers to do nowadays in the age of broken games.
There's few justifications of said cut, and we only have speculation as well as the word straight from the horse's mouth: 343 themselves. They made a statement - an official statement on Halo Waypoint I might add - saying that having splitscreen in the game would compromise the vision for Halo 5. They didn't scrape it under the rug and hope that no one would find out. They laid it out for everyone to see knowing there would be backlash. A few reps from 343 had already stated this beforehand a few weeks before I believe. Possibly around mid-summer, to late July.
Granted, we can only take what they say at face value, and we don't know for certain if what they say is really true. This is coming from the same people who shoved out a broken game last year, so ALWAYS take what they say with a grain of salt.
Now, about the actual viability of splitscreen: Halo 4's splitscreen is atrocious. Not only that, but the MCC's version of Halo 4 sucks just at much. Additionally, Halo 2: Anniversary Multiplayer also has framerate problems though I have not played it in a while so I would not know if it's been patched yet. Still, you have to wonder if Halo is even capable of running splitscreen anymore. I mean Halo 4 struggled to maintain composure not only on the 360 but on the Xbox One. However, it's a part of the travesty that is MCC so it's understandable if does not work quite correctly.
Though I'm obligated to say that I have not played a "next-gen" game with splitscreen other than the MCC. I do not know if any game in the market other than Gears of War: Ultimate Edition even has splitscreen functionality.
So I think it's just as much a question of IF the games can handle splitscreen as well as IF they even want it there in the first place. Having splitscreen in the game would mean that people would buy less Xbox Ones, so... conspiracy theories...