I think you've got a lot of it wrong, starting with a very flawed premise. The thing is that arguably "Alien" should never have been turned into a franchise, it worked well as a specific, stand-alone story. "Alien" worked because it presented a specific setting, characters, and enemy, that could conceivably come together and play out a lot like you saw in the movie. Part of what made it believable and helped create the tension is that just as the characters were flawed, the alien itself wasn't so invincibly superpowered that things were a foregone conclusion which added to the tension as people went looking for it and so on. The way the alien dealt with the crew was as a result kind of fascinating, and while it was an alien, it was very similar to what a stealthy predator like a large cat might due to people under the right circumstances...
The thing is that outside of that specific scenario the "Alien" itself wasn't that threatening and that was kind of the point. You put it up against guys who are properly prepared and trained for dangerous situations, and it's a lot different. "Aliens" deserves points for being able to use largely the same material and tell a new story doing it. If you made a franchise simply based around what you saw in "Alien" it would turn into predictable seriel-horror, playing out the same way, again and again. Our "xenomorph" would become yet another creature picking off unprepared people using slight variations on the same set of tricks until people got really, really, bored with it. To be honest if someone had just made "Alien 2" and did it like the first movie, I doubt there would have been anything else in the franchise (comics, movies, novels, etc...).
As far as "deep" characters go, I'll add as a side note that I find it kind of annoying when people complain constantly about not having them in books and movies, especially when they do not belong, and might detract from the overall work. At the end of the day most people aren't all that deep, and when it comes to certain things like military squads, the whole point of being "military" is to sort of force everyone into a specific mould, that's what Boot Camp is for, to beat down individuality and build someone back up as a soldier (which is literally a tool). A bunch of marines on a combat drop are not going to be showcasing much in the way of deep sentiment, contemplating their navels philosophically, or whatever else. Veterans moving from hot spot to hot spot in particular are going to be very repressed emotionally since part of fighting a war is to put everything that is positive about humanity in some deep part of your mind and lock it up with a key while you run around being a monster... which incidently is why a lot of combat soldiers have trouble taking their humanity back out and being normal when they come home. On a lot of levels what makes a movie like "Aliens" work is that it sells the situation by having the marines act like... well... marines.
Not every story has to be deep to be good, and honestly attaching a lot of garbage onto things that don't need it has probably ruined as many, if not more, things than it's saved. Sure "Terminator" and "Aliens" were very simple stories at their heart, but I don't quite consider them "brain dead action fare" so much as they stayed pretty much on topic. Both movies also sold some pretty amazing concepts (robots, aliens, time travel, space ships) and got people thinking about those kinds of things, at a time when science fiction, fantasy, and things like video games, had nothing like the penetration that it does now. "Terminator" for example would not have benefitted from a digression where Reese spends 30 minutes of screen time trying to live out his secret fantasy of being a male ballet dancer now that he's in the past, just to prove how nuanced he is (awww, look, a hardened tough guy and demo expert, who secretly just wants to dance...).
Don't misunderstand the point here, deep characters who evolve are fine, but they don't always work, sometimes a movie is as much about the situation as it is about the people in it, if not more so. What's more, as I said earlier, not everyone is all that deep, even without training that serves to suppress individuality.