Pokemon has never been known for its extensive post game story and replay value. The delta episode was quite special and I'm still in the camp that says the main reason behind it was a nod to Pokemon Emerald and Rayquaza needing a big role in the Hoenn story. On top of that they needed something to explain the origin of Mega Evolution and what better place to put the origin than putting it at a mysterious Pokemon who roams the Ozon layer which even today scientists haven't fully figured out.
That said, the real replay value is indeed in what you said - battling. The competitive scene moves the Pokemon community forward and keeps it alive once we are through the story. And along with battling there is of course the hacking scene which offers randomizers and hackroms and there are challenges such as nuzlocke challenges.