If they were to all be unlocked, the higher level plains, desert, etc. would be fairly redundant. If we are giving up any hope of ever having zone fights like the past then maybe throwing in some new areas for them to be named would be beneficial. After all, having two plains, regardless of the seeming utility now, is kind of pointless. Turning it into a new area and then unexcluding (is that even a word lol) it would, in my opinion, be better than just removing the cap and throwing the same old zones into the mix.
Plus, for those at the lower Oro kills, private zones seemed like a feasible alternative to this altogether. I mean, gold is pretty expendable nowadays right, especially for sitters and those already at max reborns.