Alien and Aliens are one of the best movie pairings ever made imo. Both masterfully done, each really existing inside their own sub genre, and yet they go together so well.
Alien is a suspense film. Amazing set designs, great humanistic portrayals of blue collar workers in space. You rarely even see the xeno until the end, and yet it also has one of the most famously disturbing violent scenes in cinema. The chest burster scene was unparalleled for years. The effects of the whole film hold up very well.
Aliens takes the first film and flips it on its head. We go from blue collar workers to soldiers. Both existing within a relationship with a controlling corporation, that's the constant thread, but now it's a war film. We go from barely seeing the xeno to being swarmed by them. There's still suspense, but it's not horror in the dark, it's an action film. Nothing feels repeated or formulaic, the connections between the films are fabulous but each really holds their own.
Both have a wonderful balance between the humanity of the characters, who aren't just there to be killed off like in so many more by the number horror films, but also the wonderful design of the aliens. The eggs, facehuggers, chesbursters, xeno drones, the Queen, the space jockey, his ship, just everything actually "alien" about the movies was so well done. HR Geiger was a genius, and there were two visionary directors with long careers ahead of them who made the most of it all.
I don't hate the third film, but they threw out all that development for something that never measured up. It's not a terrible movie, but hardly worth of being part of a trilogy. I'm happy to forget the fourth film even exists, it was not only not good, it took the only really meaningful thing the third film had to offer, Ripley's sacrifice, and just flushed it all down the toilet. Bleh.
The license would have allowed the third movie to be in the pin (not sure about the fourth, only remember discussion of the third). Nobody wanted it. Definitely the right call imo.