Where it all went wrong for the saga?

i never even bothered to watch 3 or 4

Fincher turned out to be a pretty great director. And he was a good fit for an Alien movie. There was a lot of studio interference on Alien 3. I wouldn't judge Fincher on how that turned out.

Ressurection was a little different story, though.

It went wrong when they dragged Ripley along in the sequels.

No way Fincher was to blame. He went on to direct such little movies like Seven, Fight Club and Zodiac. Without Alien3 those movies most likely would not have been made in the form they are today.

I think killing off Newt and Hicks immediately after the events of Aliens was sour for many people, they went through too much in Aliens just to be offed like that, it was actually kind of disrespectful to Cameron's movie as well, though thanks to ret-con Hicks apparently survived (see Aliens Colonial Marines).

Alien 3 does have its moments though, and it still feels like it takes place in the Alien universe, and it's not as intense as the first two films, and perhaps missing some of their atmosphere and immersion.

Some would say Aliens because how it changes the alien. Yet Cameron put a lot of care into making the sequel mesh with the original in terms of aesthetic, characters, and tone. The first two movies most feel like they're in the same universe. You can find something to like from any angle - the characters, the sets, the practical effects alien designs. They created a lot of positive momentum and good will with audiences.

The third movie threw that out in the opening act by killing two major characters and much of the tone/momentum of the first two movies. There's a deep space exploration (and dread) atmosphere to the first two movies. The first has the Nostromo in deep space. The second has the colony, which is on the planet from the original. The third grinds that to a halt on a drab prison planet with a bunch of bald guys who mostly all sound and look the same (to American audiences).

If they were gonna go with an "Alien on a populated world" theme they should have went all out like on a space station or colony. The way they did it, by killing two beloved characters and setting it on a prison world with characters most people don't care about, wasn't a good way for most audiences.

They could have allowed Hicks some screen time before killing him, give him survivor's guilt and PTSD to keep the dour tone of the movie, don't have to make the three of them perfectly happy. You could show them coping with the events of Aliens, and then throw them into another mess with the aliens so they have no time to process it. There's plenty of dark territory that could have been explored while respecting these characters and giving them arcs worthy of them.

What they did was absurd. To bring the series momentum to a screeching halt was nonsensical. So was thinking that the franchise was about Ripley alone and her being in the movie (and the alien) was all that was needed.

That's like saying Alien could have been made by Ridley Scott without HR Giger, Dan Obannon, and so forth. How has that worked with the prequels?

The best movies are ensemble pieces from crew to cast. Then again maybe Cameron got the ball rolling by saying Aliens was all about Ripley, even if it wasn't.

It all went wrong for a few reasons. Fox demanded a sequel to Aliens right away and Giler and Hill had no ideas on where to take it. Initially they wanted a script about the Company’s duplicity in using aliens as biological weapons, and even wanted Ridley Scott to come back to direct, but he had scheduling conflicts. They then made choice to go with Renny Harlin who wanted to do a “Planet of the Aliens” movie, but they realized it would be too expensive. A few other writers and directors came and went, with ideas being juggled around from an alien infestation of Earth, to an Alien virus released on a space station. One idea even had Ripley and Newt hunting aliens on some Blade Runner-esque metropolis. Then came Vincent Ward and his awful script about a wooden planet, which was the germ of Alien3. Eventually a producer at Fox suggested they just change the wooden planet to a prison planet, and voila. A3 was born. Fox made a release date for the film when they were still in pre-production.

Another big mistake was the executives wanting Sigourney Weaver back so badly that they allowed her to sign a contract giving her creative demands on all future films. She had three stipulations: no guns, that Ripley would die, and Ripley would have sex with an Alien. They balked at the last demand (saved it for Resurrection) but allowed her the first two.

Alien3 was seemingly doomed to fail from the start, but there were some interesting concepts in there. I think the best route would have been to involve the Company’s background and experiments more, combined with the Cold War theme of a hostile socialist bloc known as the Union of Progressive Peoples, which was introduced in William Gibson’s A3 script. Maybe even introduce a bigger, badder alien known as the Queen Mother. A lot of these ideas were later used in comic books and video games. In fact, there was a videogame called “Aliens Infestation” that was arguably a better sequel than any of the following films.

I don't think it's a "saga." It's just a bunch of sequels made to try to keep the money flowing. 1 & 2 were great. But, that was most definitely IT.

The law of diminishing returns... the studio wanted to make as much money as they could with as little budget as they could. Look at what they did with Superman 2, 3, and 4.