And perhaps they never should have tried to make it a franchise. Is The Godfather a franchise? The third one adds nothing to the table and everyone would have been just fine if it ended with Part 2.
There's a reason nobody had been able to continue the story successfully. There wasn't really a story to continue. The aliens don't have personalities and most of their mysteries had been resolved. Ripley had faced her fears and overcome her ptsd and survivor guilt. Other than retreading Aliens or killing Ripley off, what was there truly left to explore? Bond is able to vary the formula because the bad guys look and act different, they mix up the actors, they vary the Bond girls, and they have plenty of different settings. Once you start resolving the mysteries of the aliens, it's diminishing returns unless you have a really clever script, which neither 3 nor Resurrection had. Fincher did some good things with 3, but that's more a noble failure of an auteur than a success.
At least Prometheus seems to acknowledge that. It goes back to the time when the creatures are still a mystery and there's no Ripley in sight.