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.
I cannot speak for all Sonic fans, but the only time I have ever been completely disappointed by an official Sonic game is definitely the 3DS Sonic Boom game. I tried. A single level in and I wasn’t a fan. That, and of course that one runner game that gets constantly recycled through every single IP under the sun. You know the mobile game.
The only other time I’ve been disappointed is the latest Sonic Transformed because they’re going the route of having too many non-Sega characters in a Sonic racing game. Similar reasoning as to why I hate all Mario & Sonic Olympics titles. Cut the Big N and I’d probably still not like them because they ain’t my cup of tea, but at least I wouldn’t hate them.
I’m the easy type of Sonic fan to please, for the most part¹. Doesn’t matter whether it’s a game that starts slow paced like Sonic and the 7 Rings or something as buggy as Sonic '06, I’m here for a good time, not a long time. As long as the game is completable and isn’t the transformed racing series ( Riders was 1000x better ), I’m down to try it.
¹ As an arachnophobe, we do NOT bring up the Sonic Boom cartoon episode where Styx gets cursed. Nor do we bring up the fact I’m sore about not being good at Spinball on Genesis/Mega Drive.