The first subgroup you are talking about, "wants the exact old game," would be the group of people that would be satisfied by Halo X Anniversary/MCC. There's already no point in directly catering to them because they already have the (on paper) perfect game they want.
I'm sure you're going to bring up "they want to play a game like Halo 2 and 3, but they don't want to play Halo 2 and 3" like you did in a different comment, but that describes an entirely different subgroup than the one previously mentioned. That describes the subgroup that wants X number of changes in Y areas.
And honestly, the most numerous in this thread that actually takes the time to explain their reasoning is that very same subgroup. The subgroup that is perfectly okay with a game not exactly like Halo 2/Halo 3, but doesn't have the exact abilities Halo 5 does. Me personally, all I have to do is remove literally two abilities from Halo 5, and that solves the majority of problems for me. Is that result exactly the same game as Halo 2/3? There are still clearly mechanics from those games that are not in this hypothetical game and mechanics from Halo 5 that are still in this hypothetical game. Not saying that my result works for everyone, but many people in this thread has done something like what I just did and came to a conclusion that is not Halo 3 or Halo 5.
I don't care about "return to form." The only thing I want to "return" to is no X mechanic, but that reason isn't driven by mere population issues. I don't care about the population. An enjoyable game handles the population so I don't have to. I believe that removing X mechanic will create a more enjoyable game for Y reason, but that doesn't mean the population will increase directly because of it. Maybe it will, but I won't claim that.
Okay, then what do you mean by that?
I put "casual" in quotes because I could not think of a better word to describe them. "Uninformed"? "Surface-caring"? "hobbyists?" None of those sound all that accurate...or positive. Either way, I'm talking about the majority of players who play the game for the sake of playing the game, and don't care to go in-depth on player mechanics, or the extensive history of the game, or pretty much any element of game/sandbox design. Playing the game feels good, they feel good. Playing the game feels bad, they don't like it and quit. They don't have to verify a reason.
Anyone who goes even remotely in depth about an ability and claims to be "casual" by the aforementioned definition are wrong. Even having a Waypoint account and making regular posts here is more than I can ask for most players of the game.
If you're looking for statistical evidence, I guess I can't provide any because, good luck making a reasonable survey going "How much do you care about X?" when the population you're trying to get an answer from probably doesn't care about surveys either.
I guess there's the Xbox Achievement statistics? I don't know how accurate they are. According to that, only ~30% of players ever opened a Gold REQ pack. Only 26% of players played 3 games of Warzone. Only 30% of players played 5 games of Slayer.
Because X thing bothers them for Y reason, and they feel it would be better if X was changed or removed. That's pretty much how any criticism works. It doesn't even mean they don't enjoy the game, only that they would enjoy it more with that change.
Even Bungie/343i knows because they change/remove things based on that very feedback.
There's already precedent for that not being true. 343i removed things from vanilla Halo 4 into late game/MCC Halo 4 (random example: Instant Respawn), they've changed/removed things from Halo 4 when making Halo 5 (Loadouts), and they've changed/removed things from vanilla Halo 5 to current Halo 5 (Spartan Charge/Ground Pound in HCS).