[Rhun charioteer]

So we have a glimpse of middle earth's future as it marched into the fourth age. I am curious though how big did the Reunited Kingdom get. Aragorn and Eomer pursued campaigns around the sea of Rhun and subdued Harad(or the northern parts at least). But could Aragorn or his descendants have subdued all of Middle Earth all the way to the eastern sea and the Dark Land? Or would it remain beyond their grasp.

Also based on that question how did the reunited kingdom end? We know Tolkien considered Arda to be an ancient Earth so Aragorn's empire must have ended at some point or another.

Anyone want to hazard to answer these questions. They are speculative of course but speculative answers are what I am looking for.