Like I said over on the review comments, It really all comes down to SEGA keeping a consistent brand, something that was much needed in the Sonic series.
Consider that all of these appeared in stores within one year (2006):
Sonic '06, Riders, Rivals, Sonic X, Sonic Genesis
Now here is 2010:
Winter Olympics (iPhone, Wii), ASR, Sonic 4, Free Riders, Colors, Adventure also Alton Towers uses this imagery
Right there you can see that compared to 2006, SEGA has maintained a much stronger consistent brand image for Sonic in 2010. I'm not saying review scores are dependent on a solid brand image, as it really all comes down to the quality of the gameplay, but a consistent brand image helps.
Rather than thinking "oh a new Sonic game and a new iteration of Sonic! They must have failed the last game and rebooted.", consumers think "oh a new Sonic game with that Sonic from the previous game! I loved ASR, so I'll probably like Colors!". It helps too that the current design is associated with a positively received run of games. The same design that appeared in the well received All-Stars Racing also appears in other well received and anticipated games. Almost as if Sonic jumped out of the last game and into the next one. As long as SEGA keeps the game quality consistent, consumers will continue to associate the current design with "good Sonic" and the 2006 assortment of images as "bad Sonic" (Genesis Sonic excluded).
As for Classic Sonic, I'm not against SEGA bringing him back (if only for DLC or nostalgia value) but I really think that building on the current design is more important for the brand.