If Sauron prevailed...

[Inziladun]

Originally Posted by Rhun charioteer

Sauron in theory could reign over Arda for thousands of years as an immortal god king to his subjects terrible and mighty, the lord of men and the earth.

It would be cruel and nearly a dereliction of duty of the Valar to allow this state to remain unchallenged.

Like I said, I don't think the Valar would have tolerated Sauron indefinitely. A renegade Maia making himself king and god over Middle-earth was an insult to their authority, not to mention "unfair" to the lesser beings in his path. There would have been another plan to bring him down, though it would almost certainly have involved more cataclysmic damage and loss of life.