It is beyond just one guy bad decisions on a bad movie, XCU needed a "showrunner" with a long term vision. No amount of good decisions was saving Jean and Scott or Scott and Ororo from feeling a little awkward in Dark Phoenix. Now let's imagine if space rescue was at the beginning of Apocalypse. And you had a whole movie to build on something was happening to Jean and these characters friendship the next movie would have been on the sheer fact it feels like everyone cares for each other. Avengers 2 and Iron Man 2 are average movies being nice but those movies set up a ton of things that lead to successful other movies. Doing movies one by one as an individual movie is a good approach except for when you for sure are getting another movie. Dark Phoenix failed as much from Apocalypse not setting up things as did from Dark Phoenix itself being a mediocre movie.