Halo can recover if 343 goes back to the game's roots. Call of Duty just did it and I have friends who haven't touched an Xbox since MW2 was still popular telling me they might get back into it because of this game.
The community is so divided though it's a mess. A lot of people love the new evolved style of play with Sprint and Spartan Abilities and more like the classic gameplay but not by enough otherwise 343 would've done it already. They've been put in a horrible position by the community and the only way to get out is to cater to one side (which is going to hurt a lot for whichever side loses in the future).
I love Halo 5 honestly but I think people would return and numbers would grow if they just took a step backward gameplay-wise and give it a great run and gun campaign.