Removing split-sceen co-op from Halo is like removing it from Mario Kart to me, its a game mode that has been there for every entry in the series (portables notwithstanding) and its one of the main reasons I enjoy the series. You can't just remove a feature like that after six entries across two console generations and not expect people to react negatively, especially after the MCC had it too.
I'll never forget how I somehow missed that it wouldn't have split-screen co-op (Halo 5), I pre-ordered it, and when it arrived my girlfriend and I had found a babysitter, we sat down together on the couch, and then she watched me struggle for an hour trying to get split-screen to work before she eventually googled it and told me it didn't have it, and I still didn't believe her, and kept trying to figure it out. Halo without couch co-op? lol.
We then gave up and watched Netflix, but the next day I went back to it and played on my own for probably 3 hours or so, and never went back to it.
I suppose if I had more Xbox Live friends you could find some fun in it still, but I definitely prefer playing with friends/family on the couch in the same room. It's just not the same.