As for it being fated, I'd argue that despite what was said in T2, the existence of anything in the franchise after that means it was wrong. You could argue that it wasn't fate, but humans just did it anyway, though.
There's a divide between T1/T2 and the rest of the franchise. The presence of material after T2 doesn't inherently negate T2. Cameron ended his films there, everything after that is pretty much fanfiction. I don't mean that in a derogatory manner, but the divide remains. It's why almost all of Terminator media treats T1 and T2 as having occurred, but then does its own thing. SCC, T3, and T6 all occur after T2 for instance, but all are mutually incompatible with each other.
Disagree with this. Sure, Earth is apparently a mess according to Resurrection (which is what I think you are refering to), but humanity is all over the place by then and Earth is not longer relevant. And it's humanity that's the reason why.
Wait, are you disagreeing that Alien is Lovecraftian, or that it's grimdark?
Anyway, Earth is screwed by the timeframe of Resurrection, and it's implied that humans are the reason why. That isn't a happy thing, that's a tragic thing. And even if humans are spread beyond Earth, the quality of life doesn't seem to be that good.
EDIT: Argh, the thing ate my section about why the Aliens aren't really scary. Anyway, it was about how people had to make 2 really evil/stupid decisions for things to go wrong in Alien, and about 4 in Aliens (and the Aliens being really lucky in where they chose to put their nest), and even then they only are a threat because there's dozens or hundreds of them for each survivor (who are low on resources, injured, and on a time limit).
Just get one of those decisions right, and things would have gone very differently. Not saying it's unrealistic (look at the covid threads), but that it's not the aliens that are the real threat.
I disagree.
The xenomorph in Alien is definitely a threat. Even if Ash wasn't trying to keep it alive, things would have probably gone badly.
As for Aliens, I doubt things would have gone much better regardless of...well, I'm not quite sure. The xenomorphs would have got the drop on the marines regardless. If things went better, they could nuke the site from orbit, but even then, the xenomorphs overtake an entire colony in the first place.
Also Alien 3. One xenomorph is able to kill, directly or indirectly, an entire prison colony bar one person.
But the xenomorph aside, I'd argue that Aliens is grimdark, at least by my definition. Even if the xenomorphs didn't exist, humanity is still in a state where corporations wield enormous influence, where frontier life is rough, where Earth is in a state of collapse, etc. If you look at the timeframe of the Alien films, things get worse over time, from the more upbeat future of Prometheus/Covenant, to the corporate sociopathy of Alien, to the increase of that sociopathy in Aliens/Alien 3, to the squalid state of things in Resurrection. Yeah, WY is gone by then (even if in the EU, it takes over from the USM), but life still seems to be crap.
Or, TL, DR, things in Alien are bad for humans, stay bad, and never get better, in fact, they get worse as you move forward in the timeline. Terminator, on the other hand, has multiple futures where even in the worst case scenario, humanity still wins against Skynet after Judgement Day, and in the best, JD is averted completely.