If released in 99, it would bomb like Serenity, Riddick, and the countless other "Sci-fi epics" that come out every year and bomb.
Star Wars succeeded in '77 because it was as Wilhelm said, a first. The prequels succeeded in 1999, 2002, and 2005 because they were sequels. They would've failed as stand alone movies. Why?
1) The special effects would've been nothing new.
2) Sci-Fi movies are pumped out of studios every few months.
3) The plot was boring, the acting was bad, Jar Jar was annoying, and the only decent part is the saber fight.
Much better movies than TPM bomb (I again, cite Serenity). This would've been no different.