If epic pets are introduced, we will be forced to level 5-600 pets + new pets next expansion and stone them again. With current stones/hour rate in MoP, we will be looking at one years work + for the sole purpose. Players having epic pets will force all new tamers to have legendary pets only so they have that slight statfavor, and so far there isn't something after legendary and there won't be.
Im sure Blizzard want the petscene to be more fun and add in major changes, keep adding in blue quality raiddrop pets each patch isn't going to last in their eyes. People with full collection needs something to do aswell, so ye, pets to level 30, a fourth ability, gender breeding, epic quality. Something will be added to turn our worlds upsidedown. Happens every expansion so players have something to grind to keep subs running.