The original Alien came out in 1979 when I was in high school. It was a low budget film released with very little fanfare but became an instant critical and commercial success and all time classic of the science fiction/horror genre. If you proceed directly to watching Alien Romulus tomorrow night, you will see it mercilessly rips off the original, although it does have some redeeming qualities- good young actors, good action scenes. It's essentially a remake of the original with many of the same plot elements kept in place.
OK, time to geek out a little ...
Yeah, it's true, HAL9000 is early AI. Built by a certain "Mr. Langley," I may note for enthusiasts of the Deep State.
Something the plots of both 2001 & Alien share is that the human crews in both films are deemed to be "expendable" in favour of their greater missions. "Mother" in Alien is also a kind of AI, though not as sentient as "Hal."
There's a much lesser known movie from just a couple of years after 2001 that quite accurately anticipates contemporary apocalyptic fears about AI run amok called Colossus: The Forbin Project. Two super computers from the US and USSR start communicating with each other and end up enslaving humanity by the end of that particular movie. Well worth seeing if you can track it down.
I'd argue that Alien itself is, and is not, completely "original."
Everything you say about it I completely agree with, especially in relation to Romulus, which I too enjoyed but may never rewatch. Whereas the original is something I could watch again every year.
But, however brilliant in execution, it is, at its base level, a highly sophisticated reworking of such 1950s era sci-fi/horror films as The Thing From Another World: ie. the scary space alien; the eerie isolated & claustrophobic setting; etc. Almost a haunted house movie of old.
Except, Alien is an older-style B-movie story made with A-level talent, budget, vision, craftsmanship, script, acting & design, and enhanced by a greater degree of aesthetic realism and the then latest developments in special effects.
Spielberg & Lucas were doing much the same at around the same time with Star Wars, Raiders, Close Encounters & Jaws.That is, the basic story lines were all the stuff of the Saturday serials & genre movies they loved as kids.