Franchise longevity, in my opinion, isn't really worth it if the series refuses to innovate. I'd much rather have something finite and interesting than something that runs into the ground, spinning its wheels and doing the same thing over and over. Or, alternatively, something long-running that isn't afraid to take risks and do something new, when the potential is there (and Alien is a series that I feel does offer that potential).
Which brings me back to my above point, and how much I love that the Alien movies keep reinventing themselves. That's the appeal for me. And Ridley Scott, with his modern-day exploration of legacy and creation through the lens of the Alien franchise, has really hammered that home while opening up so many various paths that future filmmakers can explore if they so choose to.
I do absolutely love Alien: Isolation, and find it to the best Alien video game experience by far, but I don't think that I would want to see something akin to it on the screen, since it is already so similar to Alien, which has obviously already been done on screen perfectly.