I think it boils down to people like the idea of ripley and newt and hicks in their eyes would become a family in the sequel and later go and kick aliens asses together as a super family and be immune to getting killed and wasnt wanting the series to remain horror but action movies after Aliens.
I see this assumed a lot, but none of the unmade Aliens sequels (alternate scripts, DH comics) portrayed Newt, Hicks and Ripley as an anti-Alien fighting family. Most people say they were mad about how the characters were killed off (during the credits after one of the more infamous ass-pulls in recent film history), not that they died.
Yep, that's the reason as I see it. And definitely not the whole people just want a dumb Alien Ass Kicking Super Family to diminish any real criticisms angle.
Alien 3's opening alone is tremendously flawed regardless who dies. Killing off characters cold during the opening credits, Hicks, Newt or entirely someone else embodies nothing for the audience except faceless, empty, meaningless, emotionless deaths. Then the viewer is forced to endure the impact of these meaningless deaths as meaningful for a long duration, even though they have built absolutely zero percent of an emotional connection to the audience within this film they're watching. Who are we grieving for? We hope you watched Aliens recently! But hopefully not too many times that you think it was a disservice to them!
Such an important event thrown away in opening credit flashes is just jarring and doesn't serve your narrative the best, regardless if there was behind-the-scenes reasons why you had to do it.