After a roller coaster with soaring highs (X2, Days of Future Past, Logan) and devastating lows (X-Men: Last Stand, Origins: Wolverine, X-Men: Apocalypse), the X-Men franchise has been unpredictable at best. It's hard not to compare every comic book movie universe to the MCU, and while X-Men has been more successful than some (lookin' at you, Sony Pictures Universe of Marvel Characters), it hasn't quite nailed the dependability of Avengers-adjacent films. It's a shame that it wasn't until the mutant-film-verse was in its last gasps that 20th Century Fox decided to branch out from the traditional X-Men style and try something new. It's also a shame that the movie wasn't better.
The first "Deadpool" had a small budget (by superhero movie standards) and wasn't expected to succeed by some, who questioned whether there was a place for an R-rated comic book movie. Then, in true Hollywood studio fashion, once it became clear that the first film would surpass anyone's expectations, a second installment was greenlit with nearly double the budget. And when you're a comic book movie with that kind of money, you can bet Thanos is going to be there.
There were rumors of Deadpool making his way into the X-Men movie franchise for a while, but nothing ever panned out. After all, the character just doesn't seem like the right fit for an ensemble. Then they tried him in X-Men Origins: Wolverine, even letting Ryan Reynolds (whom the character claims to resemble in the comics) play him with... less than stellar results. Any movie that was going to do Deadpool justice would have to be centered on Wade Wilson himself, the so-called Merc with a Mouth. And surely no one would be crazy enough to......
I think we sometimes forget just how unlikely it was that the MCU would ever be so successful. There was very real speculation somewhere in Phase 2 that the market would be saturated with comic book movies or that as the franchise slogged on with increasingly obscure titles people would eventually get tired. This never really happened with the MCU, which seems to have settled on its money-printing formula. But other franchises were not so lucky. By 2019, "Spider-Man" had already made it through two iterations before succumbing to the MCU and now the "X-Men" franchise was on its last legs. "Apocalypse" had dramatically underperformed and, true to actual comics lore, the universe's timeline had become convoluted. There were still plans to keep this film universe going, but they hinged on "Dark Phoenix" being successful.
The X-Men franchise is a bit unpredictable. Unlike the MCU, which discovered a formula to keep even its worst installments from being truly terrible, the X-Men movies run the gamut from the spectacular to the spectacularly bad, sometimes one right after the other. After using "First Class" to reset the franchise and "Days of Future Past" to reset the timeline and erase past mistakes, the X-Men were once again on top, only to once again create a movie that was absolutely... fine? I guess?