That's really understating how bad environmental collapse can get. I'll spare you a lecture on hypothetical tipping points, but the biosphere isn't just something you can tinker with - if an ecosystem collapses, it can take decades alone for it to start recovering (look at the Aral Sea for instance).2023 is still the early 21st century, replicants were invented a few years before androids, and "completely decimated" is still something to bounce back from provided sufficient advancements in technology.
But even if that was the case, you'd have to explain how humanity had extraplanetary colonies by 2019, then spent the duration of the 21st century developing space travel to, um, found extra-planetary colonies.
Again, Avatar and Alien can't be in the same universe, primarily because the tech doesn't line up.In fact, Jake in Avatar watches news about species revival with cloned tigers, so that's apart of the recovery process right there. Although I think this means I should put Avatar entirely before Alien then.
In Aliens, FTL travel had been developed by the 2080s at the latest. By the timeframe of Prometheus, extra-planetary colonies existed, by 2122 (the timeframe of Alien), FTL travel, while requiring months of cryo-time, is still feasible. By 2179 (Aliens), what once took months (Zeta 2 Reticuli to Sol) now takes weeks.
Avatar takes place in 2154, Way of Water takes place in 2170. By both cases, humanity doesn't possess FTL tech, by 2169, Earth is in full ecological collapse to the point where colonizing Pandora is the only option. None of this matches Aliens.
One can have fun doing crossovers, but it's impossible for them to be in the same universe.