Are prequels doomed to fail? (spoilers-inside)

Reading the mixed reviews on IMDB, i can't help but recall the same feeling when Star Wars Episode I came out. The same feeling of fans voicing their anger and disappointment. Do you think its too difficult to make a prequel to a majorly successful film like an "Alien" or "Star Wars" and maybe they should just leave it alone? People already have too many preconceived notions in their mind of how things should be explained? I for one will admit that i was disappointed when they made the space jockeys evil. I always pictured the space jockeys as friendlys since they were sending out a warning signal to stay away from their crashed ship in the original.