Generations (and by extension Unleashed, I honestly think Unleashed is a sub-par game all-round) is also incredibly overrated and reliant on core gameplay mechanics that simply had nowhere to go after Generations. Lost World was a stumbling block, yes, but the core movement and ability mechanics had enormous potential.
The problem is, like a lot of things, Sonic Team is tone-deaf and incapable of properly parsing critique - people don't like other character gameplay because they're nearly always too removed from Sonic's gameplay and usually undercooked as a result? Make it so only Sonic is playable but invent (usually stupid) reasons to deviate into more undercooked non-Sonic gameplay (I also believe Sonic Team is paranoid that the core gameplay won't keep players' attention for long and thus try to spice things up way too much to compensate). Lost World had good ideas but flawed execution? Junk everything and rehash Generations! And so on and so forth.
Speaking of Sonic Mania, Sonic Team has the perfect fucking template for good 2D Sonic gameplay right there, and yet we see a mostly-automated and poorly designed Green Hill in Forces, making Classic Sonic look hopelessly redundant.
Sonic Team lost the damn plot a long time ago, and still haven't found it.