I am skeptical about this movie ever happening.
Ridley is in his '70s and he's got a ton of movies on his plate. Many of these will die on the vine, just like his version of I Am Legend ultimately never happened.
I suspect the Alien prequel is one of those that will die out, or at least be stuck in development hell long enough that Ridley will pass away before it ever comes to fruition.
And if that happens, you know what Fox is gonna do next...
Remake.
It's inevitable. And I dread it with every fiber of my being.