Or rather, why people percieve them as such.
It's a simple answer, really: The declined really started at 06, and was more of an unexpected fall into a pit that had a ladder out. You all just claim it started at Heroes for it being the first truely gimmicky Sonic game.
Not to say it's completely your fault. You see, 06 was actually so horrible, your brain had to rewire itself to avoid any further pain, creating a negative reflex at the first sign of a seemingly bad gimmick, regardless of the quality of the actual game.
I've come to this conclusion for this one reason: I have played Heroes, Shadow, AND Riders with a clean slated mind. They were nowhere near as bad as people said.
Heroes wasn't really suffering control wise, and half the problems with Shadow can be outright avoided for all but one or two levels that are easily skipped. Riders was the only really bad one, and that's not saying much.
Sure, none of them are perfect, but they're two standard quality and one somewhat bad quality Sonic games.
Open your eyes, people, Sonic didn't die. Your respect for him did.

(Note, while the mental reflex was a joke, you guys really did overexaggerate for a good long while before I actually saw some people talking sense.)