I would never buy an Xbox for just one game.
People lots of times buy a system because of one game. Halo CE on Xbox, Sonic on Genesis, Zelda on Nintendo (take your pick lol) My buddy bought Sega Genesis just for Sonic the Hedgehog back in the day. He Absolutely loved that game. Sure, later on he got other games, but Sonic was the reason he got Genesis. Without it, he may of never gotten Sega Genesis. I have multiple friends who bought Xbox just for Halo CE or 2 and that is the only game they even have for it lol I myself bought PlayStation 1 for Wipeout XL after it was cancelled for Sega Saturn. That was the only game I had for so long (still own it today!) Later on I bought Castlevania Symphony of the Night for it as well.
It always starts with one game. If Halo 5 is that game for someone then so be it.
@OP Me personally, I don't think I'd buy Xbox 1 for just Halo 5... But it depends on how much you like it/Halo. Maybe when Halo 6 comes out I would, who knows.... Have to see how it is.