I have to agree with I'll Begin. Specially in the 3rd paragraph: Sure, they've declined a bit, but they're still playable. I mean, nearly every recent Sonic game involved the Chaos Emeralds, if not a similar transformation.
And another thing I specially agree with is the 2nd last paragraph: All good universes need to be expansive. Look at Mario. He started as an 8 bit plataformer, and now there's stuff like stuff in the real world, and everytime in a different place. It also has it's certain bit that's almost constantly the same though: Bowser as the villain. Super Paper Mario makes a change (with Bowser as a playable character and a new set of villains), but it's still almost the same. Oh yeah, and lava. Specially lava. That's always, I repeat, ALWAYS, in any Mario game. Specially near the end. But then in the Sonic universe, Eggman's machines are always a bit too fancy-looking. And nearly every force he's unleashed has turned on him (Dark Gaia in Sonic Unleashed, Chaos in SA, Shadow in SA2, E-123 Omega, and Metal Sonic in Sonic Heroes), which also gets monotonous. But hey, not everything's gloom and doom for Sonic. He'll improve over time. They're starting to focus on his speed (because he's supposed to be fast) and a bit more platforming (from his origins), while still varying.
And my own opinion about STJr taking over SEGA, well...It'd be radically different. The gameplay would be more classic, but then there are some graphics whores who usually don't play if the game isn't top-tech, which I find anoyying. But the games would be dang awesome...
Well, maybe. I admit I might be wrong, but still. It's theoretically impossible, so we'll never know.