You'd be correct if the 85-90 zones in MOP were cross-realm zones. Per the blue, the only zones being considered are the dead low-level zones, of which there are plenty between two Vanilla continents that can be utilized to satisfactorily provide an interactive questing community.
Wouldn't surprise me if Wintergrasp and Tol Barad get revamped and turned into cross-realm PVP zones. There's your world PVP brought back.