That's not open PvP. Tol Barad is not open PvP. It's instanced PvP in every way except how you get into it. But since it only starts at certain times, kicks people out to meet the population limit, actively lets you not enter to enforce it..
Yeah, it's not "open world PvP" at all. It's functionally a warzone. And yes, that would fix the zerg problem.