It's one thing to not have splitscreen in new IP because it's becoming less important as a main part of people's gaming activities. But it's another thing to remove it from a franchise that is known and loved partly for having splitscreen, just for the sake of what, making the game prettier?
Man, has resolution, textures, lighting and particlae effects really become that important that they are preferrable over a game mode people love? If Phil said "Halo 5 resolution is 900p because we needed to facilitate splitscreen." people would have said "Yay! Phil Spencer, the people's champion!" They would not be shouting "WTF Phil!?! Get rid of effing split screen, it's gotta be 1080p or no buy!"
People would probably be fine with split screen dropping you down to 30fps in order to carry the load. End of the day, people playing split screen are mostly doing it for funsies not doing it with their serious gameface on.
So this is just a continuation of the game industry in general getting us to pay more and receiving less.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix