I love Halo 5's MP, but it's gonna have a much narrower fanbase than other FPS' just because of it's difficulty. I think one of the bigger issues is that, despite being Microsoft's premier studio, 343 really doesn't inspire any confidence.
They just ripped off CoD mechanics for Halo 4, and MCC was straight up broken, and still is. Where's Frank O Connor with that explanation of why it launched as it did that he promised? They launched Halo Waypoint, which had a bit of life, then replaced it with The Halo Channel, which basically died immediately, never mind because it lacked content, it performed horribly on XB1. Why did a video streaming service stutter and lag so much? Forward unto Dawn was decent, and Nightfall was abysmal, and what happened to that Spielberg TV show? The most recent offense being Halo 5's boring story, especially compared to what the marketing and Hunt the Truth were doing, how come we didn't get that in the game? Instead, the big thing for campaign was 4 player coop and some of the most basic AI commands seen in any video game.
I think all of this stems from the fact that 343 doesn't really have much in the way of creativity or innovation. Bungie pushed brand new features, and we've not actually seen anything "new" in Halo since they left. Look at Doom's Snapmap versus the current state of Forge in Halo 5. Sure Tom French and the Forge team are doing amazing work putting in new features with every update, but then they're hindered by the lack of a custom games playlist in matchmaking and a decent way of finding the content people are making. Even Halo 3 had the "Bungie Favourites" bit.
It hurts to say it, because I really like Halo, especially Halo 5, but even when you look at the DLC plan for Halo 5 being old features from previous games, I can't say 343 is a studio I trust. In all honesty, I only really enjoy the Arena multiplayer, and I'm inclined to say it seems to have come together as more of an accident than any skill or direction.
On top of that, the Xbox One's position is probably hurting all its exclusives, so they have less and less money to play with.