Well I guess it could have worked out if the Alien was more intelligent, but I still don't think it fits its behavioral patterns we see throughout most of ALIEN. And certainly, it doesn't fit after everything after the second film.
One thing that bothers me about PROMETHEUS with no Alien... does anybody really care about the Space Jockey THAT badly that they want to see an entire movie exclusively devoted to the Space Jockey? Is it really THAT important? I'd much rather a movie about where the Aliens came from, and throw the Space Jockies in there, show the crash on LV-426, etc.
And by the sound of Prometheus, it might be weird knowing the Space Jockies have so much to do with the human race. To me, the Space Jockey was something from far away, completely mysterious and unrelated to humans. It's like all the things the newer Star Wars Trilogy added to the series.... Darth Vader built and owned C-3PO and R-2D, he was friends with Greedo.... all these other things that just kind of add an unrealistic cartoony "cheese" to it. I hope Prometheus and the history of the Space Jockey don't do that, but I dunno.
Basically, I mean adding in all these friendships, alleigences, and connections just takes the fun out of some of the randomness in these movies.
I do agree, it sounds stupid that Ridley was saying Prometheus would be an Alien film, and then he changes his mind. Really bad marketing idea, IMO. I just hope he's pulling our legs now just to shock us.