For the record, I hate everything Modern Sonic represents.
From his horribly lame attempt to be a "cool" or "edgy" character to the "memorize or you die" and wall-hunging gameplay he's almost always plagued with because the emphases of speed over actual control and stage exploration.
The novelty of Classic Sonic was that he had attitude but, at the same time, didn't have his head stuck up his ass in horrible attempts to be serious or bad-ass. He also had the cute factor that is widely accepted in anthropomorphic characters like Mickey Mouse and such.
I'm not going to hate on Colors, but I'm going to hate on Modern Sonic as a character, because he sucks and is not memorable. Classic Sonic is a much more memorable figure. Seeing Classic stand right beside Modern is proof enough that they didn't need to redesign him, at all.