People don't talk much about Gen X or the New Mutants because everyone knows that this current iteration of the X-Men is going to be part of the MCU. You might have something of a point in there if we were just talking about some random x-branded movies, but the story constraints of the MCU simply don't facilitate the idea of introducing the X-men and also all the junior x-men and also all the junior junior x-men all at the same time. Those teams need some sense of history to work the way they did in the comics. So something else will have to lead at the start. There's nothing wrong with the idea of Storm doing that (though there can be other ways, too).
Also, the opinion that making changes in adaptations will automatically lead to pissed off fans and failure doesn't stack with the history of the MCU. Iron Man rewrote the character's personality. Ant-man/Wasp, Hawkeye, Scarlet Witch, Quicksilver, and Vision were all changed in various significant ways. Valkyrie is a totally different character from the traditional 'Valkyrie', yet one of the most popular MCU sidekicks there is (so popular, a lot of people want her to stop being a sidekick and get her own movie).
Also, the X-men is a series that's literally about diversity. The mutant problem is a civil rights analogy and always has been. It's understandable that stories written in the 60s and 70s would have difficulty tapping the full potential of that subject without coding things and making the main stars look mostly 'normal', but making a series about that kind of material that doesn't visually celebrate actual diversity, including in the cast, is a recipe for a pr disaster in the modern world. One that could dwarf the angry fans you think will appear. So even if we hypothetically get those Gen X/New Mutants films, that doesn't automatically absolve the main X-men from doing a better job showcasing a more diverse cast, too.
Most of the changes didn’t piss people off because they were third tier characters (even though they had their fan base) They did make people giggle a bit sometimes....Quicksilver and Scarlet Witch and the dancing on the head of a pin to strip them of their X-Men links.
I don’t think Iron Man’s personality was so big a change, the core character descriptors are all the same...it’s just Bobby D doing his thing and updating to fit in the modern era the films are set in. If they had cast Denzel Washington, I can imagine there being a little bit of a ripple, but no-one much minded SLJ as Nick Fury, beacause (a) it’s SLJ, (b) Nick Fury ain’t top drawer and (c) ultimates.
Thing is, Marvel didn’t have their most popular properties in their film rights portfolio when they started the MCU.
In term of diversity in the X-films etc...yeah, I kind of hear it, but it goes back to the key problem that film-makers and to an extent audiences are only interested in the big names. Whothisface Trek fan who’s name eludes me...Singer...was only interested in his personal faves when that film series started, and from the get-go he made a mess of things that would have helped further down the line (making Rogue a teen etc really messes with introducing the then popular...as mentioned XD....Gambit. All the popular X-Men of that time, are difficult to work in the world he set up with that film) and really handled too many as token appearances. (Jubilee, the viewpoint character for the extremely popular cartoon series that really got the ball rolling on the films....hence the theme tune...is a bit part that changes actress or gets cut or both almost every time out the gate.) They knew the popular characters of the time, but almost every time they work one in, they muffed it up and kept defaulting to sixties and seventies characters...probably to do with the age of Singer and co. That’s why we get mere minutes of the characters who were relevant when the films actually started being made....Gambit, Bishop, Jubilee, Psylocke....and ages with the older line up. Even then, the makers don’t know what to do with them...Storm is just side character, Nightcrawler a one off appearance. It’s telling the most recent character for them is Quicksilver.
Outside of that is Rogue of course....who they then cut from DoFP!
Everyone who isn’t the Professor, Magneto, Wolverine, or at a real push, Jean Grey (and later mystique because Lawrence blew up) is poorly served in those films.
And I have purchased every last one because hey...it’s still X-Men.
It’s telling that Deadpool...a comic character I can’t stand...is actually doing better at having X-Men movies than the X-Men themselves. Colossus is great in those. This probably because they aren’t beholden to that 1960s core.
X-Men is huge (or was when the films started) but they cocked its setup and then kept doing so. They should have maybe just gone with Giant Sized X-Men team as their base, or the nineties teams (very much the nineties teams, given when the first film came out) and gone from there. They wouldn’t even have had to try for their diversity quotas later if they had done that, because as you and I are discussing, X-Men has it’s diversity baked in from the late seventies onwards.
In short I blame Singer and execs, and the crazy love for Dark Phoenix era Jean. The films are always one step forwards, two back. Which is crazy, given how stupidly popular X-Books were at the end of the last millennium when the films started.