The posterization process existed long before Warhol. IMHO, Warhol’s fame exceeded his talent by a wide margin.
I agree with you on Warhol’s fame exceeding his talent. But he was second to none in self-promotion, and single-handedly added posterization, an effect that was originally used in advertising, to the language of 20th century fine art. (I know, fine is in the eye of the beholder). I, too, think he is overrated, but I will give him his Campbell Soup Cans and his Marilyn Monroes. Fairey reverted to the original use, but I highly doubt he had any clue he was doing so. I still think the Warhol estate ought to sue him.
Proof of that is the fact that we even know who he was.