Addressing the OP
One zone with mobs 1-50 means we don't have to go anywhere.
That doesn't mean we DON'T go anywhere, just that it's not essential. Content that takes us different places will serve us better than just redoing monster placement.
The one thing that DOES bug me in this department, is you have areas close by to your starting cities that are very dangerous except to higher rank players. The regions are too big to have just a small level range in each one; that said, we could narrow things down to like 1~25-30 in the regions containing the starting cities, 31-40 in coerthas, and some yet-to be released regions, 41-50 in Mor Dhona and future areas.

The OP seems to be forgetting that 50 is not going to be the end rank cap, and that new regions in the future will play host to the higher ranks, and not the current ones.

It's not a bad idea, it just makes something seem like a serious problem when it's not a major issue.

The game doesn't HAVE to expand beyond the 4 zones we have. So will it?
It does have to expand, unless you'd like 50 to remain the rank cap forever. So yes, it will, at a minimum when the level cap rises.