Why on Earth would you work on a project that's part of a series that goes beyond film if you think the main focal point of said series has "run out of steam"? He needs to leave if the passion is gone.
Because there are other focal points in that franchise/world that he also finds appealing, and that the "main focal point" (that being, the Alien) can be used as the most important piece to provide greater context to this new narrative – the Alien, in its perfect, form-fitting way that it deliberately defiles the human body, becomes a perfect extension of David's hatred of his human creators and his twisted view on sexual reproduction. Thematically that is still all very Alien, playing on the type of horror that has been present since the beginning, while framing it in a totally new light and using the Alien itself as the lens through which David's arc is explored. This shifts the focus to a perspective that is new for the franchise while still intrinsically linking that new perspective to the Alien. Without the Alien, David's story isn't anything. And Ridley's passion is there, in exploring that link and the lineages that he has presented; the only thing he's lost his passion for is using the Alien as the sole form of horror and simply recreating what he already did so perfectly in the original film.
And that's ok. The series is ripe for experimentation, and I'm sure whoever makes an Alien movie after Ridley Scott is going to dial things back a lot and do a movie that is much closer to what we got in Alien
to "win back" the fanbase. I accept that as inevitability, and I hope that whatever that eventual project is, it is good. I just remain hopeful that Ridley gets to finish his story before that happens.