I don't think it's a shift either. I think it's classic familiarity-breeds-contempt.
Halo 1: Wow... this is innovative, amazing.
Halo 2: Wow, OK, that campaign was a little off, but WHOA, this multiplayer component online is on another level!
Halo 3: A polished campaign, an extra-polished online multiplayer, a level editor, sharing... WOW!!! This is the most complete shooter ever! Nothing too new going on, but you'd have to be a dick to claim this is just Halo 2 with better graphics.
Halo: Reach: A sweet refinement. Nice new coop mode; well done. If you like Halo, you'll like this.
Halo 4: Solid stuff... same old gameplay you're used to. If you like Halo, you'll like this.
Halo 5: Solid stuff... same old gameplay you're used to. If you like Halo, you'll like this.
Halo 6: <guessing> Solid stuff... same old gameplay you're used to. If you like Halo, you'll like this.
I mean, eventually, you just get used to it, and unless devs find a way to add new -- and successful! -- ideas, the same stuff won't impress you like it used to. Scores just reflect that IMO.
I pray that Uncharted, for example, finds a way out of this pattern. Fortunately, they're only 2 games removed from their masterpiece in that series. I refuse to believe that they cannot do better than U2. Uncharted 3 was obviously just an Uncharted 2.5 type of effort in terms of game design.
Indies still innovate, but honestly AAAs seem to either have run out of ideas or just don't want to risk it anymore. Nowadays it seems like "let's put our linear series into an open world" is as far as they're willing to go most of the time.