This isn't a point on the superhero bubble bursting or even about the unwillingness of fans to come out for the X-Men brand, because neither are true entirely and the context of Dark Phoenix requires some slightly more specific thinking. Because the franchise fatigue for this franchise, in particular, comes from the fact that they've already killed it. Twice.
First, you have to consider Deadpool and how Ryan Reynolds' acerbic, witty superhero pastiche takes down the conventions the entire X-Men franchise had been built on. Even though they stuck him in what amounts to an annex and suggested (without completely clarifying) that he existed in an alternate universe, the Deadpool movies effectively became a fourth-wall-breaking, meta-loving spoof take-down. And once you get to the point of spoofery, the thing you're spoofing is dead. It's pretty much the rules.
And then, more importantly, there's Logan, which SHOULD have been the ending point to this entire franchise. The story was the perfect end, while offering a hint of a future with an X-23 movie: we'd seen the X-Men killed, Hugh Jackman's definitive Wolverine retired and something that just all round felt like the perfect full-stop.
To then reset and have to get back into a continuing story off the back of Apocalypse, which hadn't gone down well with fans at all anyway, was enough to give you whiplash. Assuming you could even muster up enough concern for the film, anyway.