I don't think there's any question but that Orcs, Trolls, ect. survived the fall of the Dark Lord, (just as they survived Morgoth) and continued to be a nuisance for some ages afterwards. As Gandalf says Sauron isn't the only evil power in Middle Earth, there are others even older than he, which might begin to assert themselves now that he is fallen. And there is a definite hint that the Witch King will be back - someday.
In other words Aragorn and his descendents are likely to be periodically challenged by assorted evils for some time. Though any future 'Dark Lords' will be, or at least have once been, Men not fallen Vala or Maia. Though Dark Elves are not an impossibility either.
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