My immediate answer - Sonic fans.
I don’t really know what Sega and the Sonic Team can do anymore to make these fans happy. They’ve revisited the classic formula a few times, they tried going modern routes, they tried 2-D sprites in some entries, they try going full 3D and whatever. Nothing ever seems to be enough and that’s even when they’ve tried listening to the fanbase a few times.
Nothing seems to matter, regardless. The fans will find something to be upset about with Sonic. Anything.
Hardcore Sonic fan here: It’s not that Sonic fans are hard to please, it’s that it’s a fandom that doesn’t have as strong a sense of consensus as a lot of other fandoms. There’s a few obvious favorites in the series (S3K and SA2 mostly) but even then not to the point where people get to thinking not liking them is objectively wrong (vs., say, Kirby, as someone who likes most of the series but thinks Kirby Super Star is an awful game.) There isn’t really one thing that “every” Sonic fan really wants.
This results in outsider commentary from people who are used to fandoms with strong consensus invoking the Goomba Fallacy all the time.
Hell, I’ll also note that I sometimes see people say that Sonic fans are too easy to please because a lot of us liked something like Frontiers in spite of it’s miryad of flaws.