I have no issue to take up with Lucas. Love the man and his work. There's absolutely some good, even great, Star Wars stuff being made now in the post-Lucas era - but the core of Star Wars, to me, is intrinsically that to have come out of Lucas' time calling the shots.
As for Scott, his situation is obviously a different than Lucas' in that Scott didn't conceive of the Alien franchise the way that Lucas did Star Wars. He doesn't "own" it (though he is the one that undoubtedly made the film what it is; with anyone else at the helm at the time, I don't see us getting the masterpiece that is Alien in anything resembling the form that it exists in now), and a major part of the appeal of Alien as an ongoing franchise is the way that each filmmaker that comes in tends to really put their own stamp on it. Guys like Ridley Scott, James Cameron, David Fincher, and Jean-Pierre Jeunet are all such radically different filmmakers. That being said, I also don't take issue with Ridley Scott "locking down" this era of the franchise (Prometheus is an interesting mess, and I adore Covenant) and shepherding new voices like Fede Álvarez and Noah Hawley. Inevitably, there will come a time where we will be in a post-Scott world and the franchise will continue on beyond him (and Hell, even while Scott is still around and producing, we're getting visions that go against the grain of his ideas, given Hawley's comments about Scott's prequels), but while we have him around, I'm very happy to see him continuing to oversee the series to some extent or another. Would love it if he had one more film in the series, as a followup to Covenant, up his sleeve to personally direct as well.