So, as we've seen with filmakers this talented, it only takes 1 film to get the audience 100% invested, not three, so no worries there. The reason to do an O5 film is the same exact reason to say they have past adventures, but with the added bonus of following the show don't tell rule. Otherwise, why have there a previous X-Men team at all? What is that necessary for?
So hypothetically you get the audience invested in a story about the O5 by doing an awesome movie featuring them, one that defies expectations. Then what? You basically have to get them to care about the new team in the sequel all over again. So at the end of the day, what was the point of that first movie? Do you not see why that is a problem?
In the comics, the O5 started out as teenagers and then we got a time skip and they were adults. In a comic book this is easy and relatively fast to do (depending on the artist capabilities). You can produce more content more quickly and cheaply, whereas movies take millions of dollars and two years at a time to produce. So given that, is it really a good idea to devote massive resources to make an original five teen X-men movie, then do another movie with them as adults, and then finally get to the characters the majority wants to see anyway? I don't see the bonuses or the benefits to that.
I like the idea of having an original five X-Men team that existed before; because it allows us to skip unnecessary origin crap, and gives a seamless way to introduce Beast, Angel and Iceman into the fold and make them instantly relevant and important to the world. *Not* doing an O5 movie works better in this regard, because I think the concept of the original team ultimately works better as an off screen movie (pre-history) than one that is actually produced.
The great thing about the MCU being so long running is all I have to do is point to SMHC, and we see how minor and irrelevant complaints about race-changing are. Once a good movie comes out, it's like they never happened. No alienation, all win, except for that one guy in the Spider-Man forums that everyone calls a troll.
Who did they really race swap in Spider-Man Homecoming that anyone would care about? The most notable flirtation with that was Michelle calling herself "Mary Jane", but that was an obvious cop out by Marvel (they didn't actually commit to making her the Mary Jane of the comics).
They certainly didn't do anything in that movie that would be the equivalent of making Jean Grey a black woman (or replacing her spot on the O5 team with Storm). So I don't see how that proves your point.
Another important side effect of doing the O5 as their own movie and then replacing most of them is that you set the precedent for characters to cycle out of the franchise, giving you an opportunity to tell a more dynamic story, without feeling like you HAVE to keep Nightcrawler on once you bring him on the team.
I think establishing the idea that there was an original X-Men team in the past already sets up the idea that team membership can change.
And while we have have had many X-Men movies, we've also had many Spider-Man movies, and the MCU decided to start Spider-Man before he became a master of his abilities and an established superhero. He basically started where every X-Student starts when they show up to the mansion: I have these powers, but I think I need someone to show me what to do with them. It worked for them in SMHC because previous X-Men movies don't make people care about the new X-Men. No amount love for Tobey's Spidey makes people care about Tom's spidey, if anything, those previous movie create a set of obstacles and problematic preconceptions. Your X-Men origin movie has 18 years of X-Men movies to beat in order not to be dismissed, but it also can't try to cover 18 years of story at once, because that will be compared to the Fox Men history that has a lot more emotional resonance because of it's 20 hours of content.
How were we introduced to Tom Holland as Spider-Man? Was it an origin movie? Was it in Spider-Man Homecoming?
No.
We were introduced to him in Civil War, in ONE scene where Tony Stark recruits him. In that ONE scene we got introduced to Aunt May, got to see footage of him in prior action on Stark's phone, got to see his webbing, and even got him to tell Tony (and us) about his origin, all in the span of ten minutes. We were then treated to another awesome ten minutes of him in his new suit fighting Captain America's team. All a grand total of 20 minutes, and Marvel used Spider-Man far better in that time than Sony did with both Amazing Spider-Man movies.
If we can set Spider-Man up like that, why couldn't we introduce Beast like that in an future X-Men movie? Or Iceman or Angel like that? And why wouldn't it just be better to introduce them that way, rather than doing an entire movie about establishing their origins?