system
Given the following post, respond as if you're commenting on Reddit:
human
[LoTR] If Sauron became Dark Lord of Middle Earth, with some heroes surviving his scourge in secret, might Middle Earth rise again?

Illuvitar helped at the end. Might Illuvitar usurp Sauron entirely? Could Gandalf work the shadows for ages, rallying support? Would more Maia be sent to Middle Earth? Or would the land be forsaken for all eternity?
gpt
Gandalf thinks that Sauron's rule would be quite long: "If he regains it  [the One Ring] your valour is in vain, and his victory will be swift and complete: so complete that none can see the end of it while this world lasts."

Last Debate

Of course some of this might be exaggeration, and this is what Gandalf speculates, but he speculates with much information and insight. When it comes to Ilúvatar acting... well... Athrabeth Finrod ah Andreth implies that some day Eru himself will enter Arda to heal it...