Not only does that make 0 sense in terms of actual time travel (Surely Sonic would just disappear in Sonic CD if the length of time he travelled back in affected his being), but I want either ALL or NOTHING. SEGA aren't going to tease us with a classic Sonic in 2D only to bring back them switching between Modern Sonic again. Generations should be a one off- they don't need to keep switching them all the time. They should make a game as either Classic or Modern and leave it at that.
Also, inb4 it's a game story and principles of time don't matter.