Ehm, OK, basically the idea was a Sonic clone, but it's a little difficult make a clone of somebody who will be borned in 35 years :P Meaning that if Shadow was supposed to be the "Ultimate Lifeform", then Sonic is the real one, because he has exactly the same abilities, but natural...
But I think that Sega revived because the fans wanted it. No kidding, the minority of the people is just the one who is in the on-line Sonic communities, so you don't know them. I have a friend that only wants to buy the game of Shadow because it has Shadow, still knowing that the game is bad... And the same case is repeated several times... So Sega though that Shadow was a good character... C'mon, at some moment, before the release of SA2, I though that Eggman turned good for attack to Shadow XD
So, the next time we flame Sega for doing the things wrong, ask why are they the things wrong...