It wouldn't be the phenomenon that ANH was (even beyond issues of quality) because a more ambiguous film about the decline of a great entity due to corruption and governmental problems was already being told much better in films of the Hollywood Renaissance and in the minds of many was occurring in real life at the time. Of the many reasons Star Wars became a phenomenon was because it was surprising escapist fare that didn't try and remind everyone how terrible the world was.

Had TPM come out in the same tone, character focus, dialogue, and plot it would likely have been considered another schlocky science fiction movie, with no more resonance and impact than countless ones that came out in those days. In fact given the budget necessary more likely it would have been a bomb that seriously damaged 20th Century Fox and relegated Lucas back to small pictures for good. At best it'd be considered a curiosity with some interesting ideas (lightsaber, the force, interstellar planets) that ultimately didn't add up to much.