In today's blue and blurry edition of Speak Up on Kotaku, commenter Monsieur.Froid suggests that Sonic the Hedgehog worked much better as a console mascot than a multiplatform star.
I think one of the biggest problems with Sonic and the current Sonic games is that he doesn't represent anything anymore. In the 90s, Sonic was a symbol for Sega. He was the mascot for the Genesis, moved on and tried to keep the Saturn afloat. The arguably last great Sonic game was Sonic Adventure for the Dreamcast, which was, again, a Sega system. He was their exclusive. Nintendo had Mario, PlayStation had Spyro and Crash, and Sega? Sega had Sonic. Sure, there was the character from NiGHTS, but how many people remember that in comparison to who remembers Sonic and Mario?
Nowadays, Sonic is just a character left hanging in the wind. Sony has Nathan Drake, Ratchet and Clank, Sly Cooper, Solid Snake (who happens to be more fluid in terms of mascot allegiances, but still really stands strong with Sony) and Kratos. Microsoft as the Gears of War guys and Master Chief. Nintendo still runs strong with Mario, Samus and Kirby. Among all of these, we still have Sonic, and while he's played on all of the systems, it seems like Nintendo has made a home for him on the Wii.
Sonic isn't his own character anymore. He's just...a latchkey kid.