Quality control would help. Aka, releasing only 3 games per generation (but making them really good) rather than releasing 10 games per generation and having half of them suck (and the other half being decent at best).
The other part is that running around and collecting rings just isn't fun...
This video has a detailed explanation of how Sonic changed over the years and why it faced the problems it did.
Basically, Sonic's emphasis on pure speed worked very well for 2D games, but in 3D games super-fast isn't always good. It makes people dizzy and it doesn't allow the player to make...