Well, I don't see Arena going away, just like hard modes.. they will still want to provide a challenge for the more hard-core player base.

But the game is about catering to the masses; look at the relative success compared to Everquest, which had a 95% market share for the genre. Rated battlegrounds is for the masses; I'd be somewhat surprised to see the non-rated BG's as that active really. I'd tend to think players will just queue for rated instead of non-rated bg's, as the game is the same, but you get much better rewards over time.

Arena will still be somewhat balanced, but rather then the primary concern be Arena and Raids, it will be Mass PvP and Raid balance.
At least that's my suspicion.