Considering how far games have gone and where this game is going, it's in a weird spot.
I sometimes visit a store that sells nothing but classic games as far back as the NES. Every now and then a 9-year-ol kid will pick up a NES controller and try to play a game on display and then give up becuase the graphics are lame and it's old. Heartbreaking, but we as adults can be guilty of the same thing. Someone my age criticized Minecraft a while back for being to blocky and low-poly, and yet grew up playing the same classic games as I did.
The point being is that it's easy to judge a simple game like Sonic Mania based on the standards of other games we enjoy more. Our tastes evolve and change and naturally we expect a lot of rich content in games nowadays. But I don't think that's what Sonic Mania is going for. Ask any kid to review Sonic 3 & Knuckles and they'll say it has little to no replay value. Since when was replay value a thing back in the 80s/90s? But it's all we had. Sonic Mania just might be an over-glorified nostalgia trip, but if that's all what I really want from it then I'll be satisfied. Might be niche to ask for a game but that will suit me.