This is actually a pretty similar phenomenon with what happened to the 3D titles over time. After 64 and
Sunshine, SMG and especially SMG2 started trying harder to make each stage work like a level from the 2D games. Culminated in
3D Land where they stopped even trying to pretend that they wanted 3D games to be any different in essence from the sidescrollers.
And it's not like
3D Land and its successors were unenjoyable or badly designed in and of themselves. It's that you start remembering that those other parts of the 3D titles - the openness of a main hub, the "choose your order" for tackling stage goals, the many possible approaches and paths in each stage - those parts seem to be destined never to return.
And it's not like they couldn't put out an open, SM64-style 3D game alongside a more linear one. It's that Nintendo actually seems to think they're scratching that itch already just by adding a Z Axis, conflating that with the non-linearity that happened to accompany it earlier on. Just like they think they're satisfying the
Paper Mario fans by simply making another game with the paper aesthetic and visual gimmicks, believing it was just the art style that people liked - not the depth of characters (heh) and mechanics and world building from the earlier games.
Also illustrated by their answering fan demand for another Metroid Prime... with Federation ForceIronically, given this refusal to acknowledge the series' pasts, their adherence to prior formula is what's really hurting them the most right now. They're hitting all the right notes (they seemingly believe) that came before while stripping out the accidental (I increasingly believe) quirks.
But it's kind of like
that one episode of Arthur where the whole gang bakes these amazing cookies to help Muffy win a contest. As they make them, it's clearly just off-the-cuff fun, each character contributing something to the mixture, and the result is as inadvertent as it is tasty. When the cookies are a big hit and she tries to recreate them by herself, based on her own incomplete knowledge of what made them good in the first place, they're terrible. They were only ever good
because of the quirks and flavors (i.e., "unnecessary fluff") inserted as part of the act of initial creation.
With
Paper Mario, creation is no longer what is occurring; rather, it's just an attempt at
recreation. Following a recipe which they only think they've committed to memory, but losing sight of the ingredients that made the first batch delicious.