Halo was stagnant since Halo 3. Halo 3 multiplayer is awful, single player is great with a couple terrible levels throughout. Halo Reach has awful single player and awful multiplayer.
Halo 4 felt fantastic to play, but the class shit was terrible, and the killstreak option of dropping in weapons or a buff was terrible. Halo 5 is seemingly great but not that many people care about Halo anymore.
Even if mistakes weren't made, Halo's popularity dwindling overtime was a given, with how each game doesn't change that much. Halo needed to become Battlefield, large scale battles, 32 vs. 32, more vehicle variety. Instead its been the same thing since Halo 1 when it comes to design.
Bungie's design philosophy is also fucking stupid, always has been after Halo 1. So much pussyfooting when it came to the game's speed, the damage weapons do and how they handle, vehicle damage and how vehicles handle. Their work on Halo was always a mish mash of ideas that either worked or were terrible.