I have a possible reason why the oldies are here.

You see, they're surely gonna make another GBA Pokemon game. I could bet. so if they release more of old pokes, this makes chance for..

trading them back in sort of a time capsule!!!!

yes, we'd be able to trade in our R/S goodies to the next gen games, and vice versa! if they release new Pokes in the next gen, I think they'll be like in GSC and RBY. we could not trade them back if they'd be new or with new moves which are possible to be in the next gen games too. Makes the thing with the unobtainables clearer aint it? *hope hope*