I'm pretty sure K-A got retired in order to make way for E, especially since they mean the same thing.
As for Sonic games getting more serious... it's pretty retarded, I've found. The storylines are overblown and trite, the settings have almost NOTHING in common with the levels in the classic Sonics and the characters just seem so lame when they're all "OMG THIS IS SERIOUS DRAMA" especially since the majority of the characters are cartoon animals.
Though, to be honest, my biggest problem with the recent Sonic games is not the "maturity" it's the fact that, well, they're terrible. The 3D Sonics: Adventure, Heroes, etc. just aren't fun. Sonic Advance and Rush, on the other hand, maintain the classically fun gameplay of the originals, while still having the "cooler" looking character designs, but without the crappy storylines and "serious business."
In short... I like Mario and friends JUST how they are, and would be horrified if they went down the Sonic path.