I'm all for the franchise moving beyond Ridley. The whole appeal of Alien as an ongoing franchise is that each new filmmaker that tackles the material approaches it in a new way and puts a new spin on it.
But I also think that not letting Ridley wrap up his story with one last installment in his prequel narrative before moving on to new iterations of the series is a colossal missed opportunity.
I want to see what new people do with Alien moving forward. James Cameron, David Fincher, and Jean-Pierre Jeunet all brought things to the table that I adore, and helped shape Alien as a sort of "auteur's" blockbuster series where the filmmaker's voice really does allow each installment to differ from the rest (even if Fox sometimes tried to squash that voice). The franchise on the whole isn't Ridley's. But the prequel story is, and the prequel story remains unfinished.