Squirtle should never evolve. Ash doesn't need a Wartotle.
Squirtle's personality is why it's so popular, why would they kill it off for a pointless evolution? It's the whole point why people like Bulbasaur and Squirtle, those two evolving would be idiocy upon the writers part. Squirtle kicks ***, I don't know why people think otherwise, it's stronger than that dim-witted lobster anyday.
As for his Johto pokes, yeah maybe some of them should have evolved. I really wanted Phanpy to evolve, but after all this Donphan crap the writers have shoved at us (Donphan filler, then Tetsuya having Donphan), seeing Ash with Donphan wouldn't be that special anymore.