It was, to be fair, a standard platformer with no innovative mechanics that was mostly known due to being one of the only ones on the personal computer at the time which lacked the hardware acceleration to normally make it possible.
The Franchise has no identity other than the characters involved.
Sonic 4 and Sonic Generations did not merely bring back the characters of the older Sonic games, but the innovative mechanics that are still fondly remembered.
Yeah that could be a weakness (no built-in audience waiting for the next chapter...) or a strength (you can really take it in any direction if you want it to...). However my opinion is broadly the same as yours - there's not enough meat on those bones to warrant a sequel.
The Franchise has no identity other than the characters involved.
Sonic 4 and Sonic Generations did not merely bring back the characters of the older Sonic games, but the innovative mechanics that are still fondly remembered.