FOX trust him, its as simple as that.
Well he wasn't left alone with X3 or apocalypse and he also wasn't left along with DOFP since that movie had the same director as Apocalypse.
I was listening to a podcast with Jeff Goldsmith a couple of weeks ago who interviewed simon kinberg back in 2016 and he found out some early script details about the climax of apocalypse
1. Originally when mystique was trying to talk some sense into Eric she was gonna talk to him like a soldier because that was the manner in which Eric has always encouraged her to be, and that was changed because they felt the film was meant to be about family and wanted to get that vibe across.
2. Quicksilver was meant to give a much bigger hint to Eric that he was his son by after he and mystique try and talk eric down and fail Peter was to sing the song that eric sang to his daughter earlier in the movie which Eric would then asks him where he heard that song, to which peter would reply that his mum heard it from his dad and used to sing it to him as a child.
That was changed because Singer apparently felt eric shouldn't find out that peter was his son because he felt they should stick to concluding relationships set up over the trilogy and there was also a concern it would hurt the tension of the climax
Simon Kinberg was apparently in 2 minds about that because while he got where Singer was coming from he still felt it could have worked.
3. the exchange between eric and apocalypse after eric betrays him was changed from
Apocalypse: What are you doing?!
Magneto: Doing what i couldn't do before. protect my Family!
to
Apocalypse: You betray me?
Magneto: No. I betrayed them.
The line was changed because mystique and quicksilver got the family lines trying to talk him down.
I personally think the original ideas sounded better but changes happened for right or for wrong reasons like with an movie.
None of these would have bothered me either way. I didn't care whether or not Magneto found out about Peter, and the "No, I betrayed them," line was better, anyway, than Kinberg's line.
I have a problem with Apocalypse due to the same problem with X3, and that was the script. In Apocalypse specifically, the writing treats the audience like we're idiots.
1) The WHOLE WORLD has officially accepted mutants? And the only outliers to this are illegal organizations like cage fighting? We can't even get certain places on Earth to accept women...but every place is a-ok with a blue guy teleporting around a mall? No.
2) Magneto killed more people than the main villain. But because we see the destruction porn at a distance, we're too stupid to know that those dots flying off the bridges families in cars who died because Erik was having a bad day. This ruined the character for me.
3) The characters reverted to their FC status. Charles being naive in FC made sense--no one knew how humans would react to mutants and he's had a sheltered upbringing. 50-year-old Xavier would know better. He's a telepath, for Christ's sake, with mutant children who have come from different upbringings. And Erik...God. Fassbender did his best, but it's like having Bruce Wayne lose his parents, become Batman, be Batman in movie 2, and then--for no reason at all--decide not to be Batman in movie 3, have a family who--once again--is murdered and then repeats the same story arc as before. Rinse and repeat?--so, so unoriginal.
Bottom line, Kinberg did a great job with DOFP. The reviews were excellent, the box office was almost as good as The Winter Soldier, and it was well received by most fans. But when he can't conjure up enough conflict, he attempts to artificially create it by killing off characters and thinking that complex = depth and death = drama.
Him having total control of Dark Phoenix is just terrifying. I really hope I'm wrong here.
