Alien doesn't need stars. Fox should let Ridley make the film he wants to make. He's always been very good at casting unknown actors and actresses and often drawing from them their defining performances... Sean Young, Daryl Hannah, Eva Green spring to mind.
And who said the prequels are going to CGI heavy? Scott has thus far never gone overboard on CGI, he's used it to augment set construction more than anything else. He tends to like shooting through the lens. I think perhaps a few hints that he's in competition with Avatar are leading people to jump to conclusions.