Ok. But none of those x-men characters were out of the picture without a clear reason. Most of those characters still continued in Dark Phoenix, they were just played by the younger actors rather than the older - but all the actors were supposed to be the same characters (which is the entire point of DoFP). (Plus, there is an explicit and plot important explanation of what happened to the old X characters in Logan.)
The difference between Fox and Marvel is not that fox 'dropped' all these characters out of nowhere, it's that the Avengers movies worked and Dark Phoenix didn't.
Continued in Dark PhoeniX, set in the past, basically another prequel to the ending of Dofp. Really who wanted that aside from Kinberg who was clearly not fit to run a Cinematic universe let alone direct a film. And really why do we need a teen Cyclops, Jean and Storm after the old versions were resurrected in Dofp.... that didn't make sense yet Hugh Jackman and Patrick Stewart got to headline one more film?
its very clear that the public wanted the X-Men to stay in the past and be played by different actors.... just Look at the boX office numbers of Apocalypse. It couldn't even outgross X1 in North America. And the newer cast can't even save Dark PhoeniX from bombing.
And what the heck happened to Rogue, Iceman, Kitty Pryde then? And having another movie set in the past isn't an excuse to ditch Jean, Storm Cyclops, Beast, Colossus without a clear reason, they were all alive in the new timeline at the end of Dofp. The thing is if they actually care about the characters and the continuity... this would have been easier to follow and possibly with better results.
But nah, Jean, Cyclops, Beast just let Xavier live in a dumpster because they have a movie set in the 90s anyway. That makes a lot of sense.
I do think the main motivation for the last 2 X-Men movies being set/stuck in the past is Kinberg's fiXation with McAvoy, Fassbender and Jlaw. He cares about the actors more than the characters or else, Storm wouldn't utter four words in Apocalypse and Dark PhoeniX. Characters like Psylocke, Angel and Jubilee would have been put to good use especially they were retconned. while the retread for Dark PhoeniX is simply Kinberg retake for his first X-Men film as a writer, which Turned out to be worse.
If those films were presented to Feige, most of them wouldn't be approved because even on paper, they already sounded like a mess.