I think Sonic has 3 eras:
1. The Classic Era- Sonics 1 through 3 and Knuckles, CD, Spinball, MBM, pretty much all Genesis/GameGear/Saturn games
2. The Modern Era- came after Sonic was given the green eye; Adventure series, Heroes, Advance, ShadtH, 06
3. The Dark Era- Unleashed, Colors, Rush (probably one of the only decent games in the Dark Era), storybook series, Sonic 4
Okay, now it's time to analyze. Sonic was strong during his original classic days, and rose to power in the modern era. But then, he fell apart. Sonic 06 marks this fall. Sonic moved from 3D Adventure/obstacle to a rushed/running 2.5D experience.
In the Dark Era, Sonic is not given a defined role. He has contrasting illogical stories (Sonic 4) that attempt to reinvent the classics by making them something else than they are. In reality, the classics are better. The Modern era has been completely wiped out and is likely not to return because of the failed epidemic of Sonic 06. Thus, the dark ages are among us!
Shadow's death in SA2 brought us a foreshadowing of Sonic's death in the Dark Era.