Good question. The best answer, of course, is that the Reunited Kingdom (“RK”) will never fall. Of course not! It remains in its idealized form forever, an exemplar of wise monarchism, military strength and domestic tranquility for as long as humanity reads Tolkien’s works. At least, that’s how I like to think of it…
But I will play by Skald’s rules, and note that most monarchies in human history have fallen for just a handful of reasons:
Overthrow. This could be relatively peaceful (like Germany after WWI) or violent (the French Revolution). In a May 1964 letter, Tolkien wrote about his unfinished and abortive LOTR sequel, “The New Shadow”:
*Since we are dealing with Men it is inevitable that we should be concerned with the most regrettable feature of their nature: their quick satiety with good. So the people of Gondor in times of peace, justice and prosperity, would become discontented and restless - while the dynasts descended from Aragorn would become just kings and governors - like Denethor or worse. *
If you’ve got a king like Elessar, monarchy is a great form of government, but you can’t be sure of always having such a wonderful fella on the throne. One of Elessar’s successors would inevitably be incompetent, crooked, foolish, crazy, despotic, or evil, or some toxic combination thereof. Over the long term, that’s inevitable with any monarchy, especially one that actually has power. And, sooner or later, he (or she) would very likely so piss off the people of Gondor that they would rise up, depose him or her, and either end up with a strongman, establish a democracy (unlikely so early in human history, but hell, there’s plenty that’s ahistorical in Tolkien’s writings) or invite in another monarch - from Rohan, say, or Rhun, or Harad, or who knows where. Sooner or later that dynasty, too, would screw things up enough or so change the polity of the kingdom that it would no longer be recognizable as the RK.
War. From its very outset, the RK is at war in the South, reconquering Umbar and Harad, and in the East, cleaning up the remnants of Sauron’s realm. Still lots of bad folks out there. Sooner or later, over the years, long after Elessar has left the throne, one of those wars will not turn out to Gondor’s liking, and it might be conquered in turn. The new boss won’t want the old boss still on the throne, and some not-so-worthy successor of Elessar might find himself both jobless and headless.
I hate to say it, but the longstanding friendship between Rohan and Gondor wouldn’t last forever, either. Neighboring kingdoms tend to look enviously or nervously at one another over the long haul, and sooner or later they’d probably come to blows.
Dynastic failure. Inbreeding or the failure to produce a widely-accepted heir, or one old enough to actually rule, can easily lead to the collapse of even the most time-honored royal family. Such might be the fate of the House of Telcontar, and thus of the RK.
Coup. A general gets too big for his britches, or a prince or duke decides he’d look better on the RK’s throne than the current loser. He pushes off the incumbent and gets crowned himself, or rules without a crown like Oliver Cromwell, or has a puppet king for window dressing. Once the precedent is established, lots of other people look in the mirror and see a potential Supreme Leader. Upheaval, civil war and foreign intervention all possibly follow, and then it’s sic transit gloria mundi, baby.
Other catastrophe. A big volcano, meteor strike, plague (for which there is precedent in Gondor’s early history) etc. could all so devastate the realm that it ceases to exist.
Depressing. Which is why I still like my first answer best.
