It's Singer's fault. He cast people much older for his own reasons, and that's what a lot of fans have grown up with.
...sure, I got time.
They did follow my logic. When they backstoried in the characters, they kept them as unimportant supporting characters, which I've repeatedly told you was an option. When they wanted to make them important, they had one successful storyline, Black Widow, where they gave her a full herodom journey, the same as they would for a new character, in Winter Soldier, so that now she can have character development people care about. They also had a less successful storyline with Hawkeye, where they simply expected the audience to care about him and his family like characters that had been on a journey with them, and that storyline was widely criticized, and did not draw a greater interest to the character.
There's nothing wrong with having Beast/Angel/Iceman as unimportant supporting characters, and skipping on having the audience invested in their story. Like you said, you are comfortable with Beast being an unimportant supporting character with no meaningful life outside of how he contributes to the main characters, like in X3. Many would be satisfied with this. I've shared this with you before. Do you recall that logic?
I like this angle, but that's a LOT of characters, and I personally wouldn't be interested in seeing Rictor as a founding X-Man. Maybe save Rictor and Kurt? OR keep them as supporting characters. Plus your squad has no muscle!
Nope. They graduated in issue 7, suggesting they were adult teenagers (18ish) at the beginning of their career, and advanced through time normally with the rest of the Marvel Universe after their reprints. By comic book sliding time, they didn't become 10 year veterans until Morrison's run.
Okay, so there's two things here. First, thieves are real, every audience member that owns a lock is already invested in the idea, and it is a deeply ingrained action movie trope. The heist genre is a lot older than the superhero genre, and so the audience isn't surprised that the thieves in O11 don't seem magical, or that people aren't in awe of their presence, because they just aren't a big deal, as is reflected in the reaction of their opposition. Whether they are the best doesn't come up, because it's not important. They become a big deal to everyone, including each other, through the film and Danny learns that his quest for vengeance, which seemed like a big deal at first, is not in the end. It's a great journey with great contrast. In just 10 short years, Marvel has managed something similar in SMHC, where Peter having a costume and powers isn't a big deal. He still gets no respect in or out of costume, even from the likes of Happy. He still is ineffective and weak. Being a superhero, for Peter, isn't a big deal. He wants something that IS a big deal: being an Avenger. He becomes a big deal, and learns that being an Avenger is not a big a deal as he felt. It's just good storytelling, and even if you disagree, it's what the MCU does so we can count on them to do it more. I mean... why do you think people care about movie characters at all? What happens to them, whether they do good or evil?
Second, that's a really good point, we are not going to be able to do all the major X-Men storylines, nor are we going to be able to do justice to all the characters. So how do we decide which ones? How do we think Feige will decide? You bring up a good point about storylines that haven't been done already should take priority. I think there's something to that, and I think that's part of why the O5 will not exist, or not exist in the way that we'd like, because the storylines that depend on their character development: Dark Phoenix, Cable, angsty Blue Beast and Archangel, these storylines have already been done recently, and so maybe they just shouldn't happen in the MCU, or shouldn't be something the audience is intended to care about or be moved by. That's a little sad as an X-Man fan who wants a narratively faithful adaptation, but it's not necessarily a bad idea, at all, and seems to fit with how the MCU is handling Spider-Man. If I'm eagerly awaiting Tom Holland to face of against Norman Osborn over the Death of Gwen Stacy in the MCU, I'm probably going to be SOL.