Officially, the DP series was "Ash and Dawn's story" - as written in the intro text on every opening theme in the Japanese version - while the others were just "Ash's story"
So yes, it IS Dawn's story too, for the DP series... and while they didn't label it so officially, a good part of AG was also dedicated to May
Anyway, Pokémon is mainly, before anything else, about... well, Pokémon
If they'd ever want to replace Ash they can easily do it
I'm not even against the idea, though I'm not really supporting it either
I'm happy with Ash staying and I don't understand people who want him to be replaced, but I also don't like to see people who act as if replacing him is SO "impossible"... it's unlikely to happen, but not impossible to do if they wished to do it