Well, I am a huge Marvel fan. I'd love to see them in a shared universe, but I don't think it's a net positive. Marvel got the ability to produce a Sony movie and immediately pushed back Black Panther and Captain Marvel. An X-Men movie will push them back even farther. And that's assuming they don't produce a New Mutants movie, a Deadpool movie, etc. If they do that, you can kiss half the movies Marvel makes goodbye. I like Fox keeping the X-Men because it creates more Marvel movies.
Would I like the two to be able to reference each other and be part of the same universe? Sure, it would be nice. But unlike the Fantastic Four, I feel the X-Men stuff stands on its own better. And, unlike Spider-Man, I think there's greater potential for spinoffs (ensuring they make at least a movie a year). Hell, I want them to be more ambitious. I want a Starjammers movie, a Longshot movie, a Dazzler movie, etc. While less likely, I know it has no shot if the rights revert.
As you pointed out, Hugh Jackman is leaving, so his desire to be part of the MCU is less significant than you think.
Selene, Nimrod, Mad Emperor D'Kann, the Brood
I think the downside is that they'll redo the movies Fox already did instead of making new ones. Do I think they'll do a better job? Yeah, I do. I think Dark Phoenix could certainly have been done better. But that doesn't mean they'll do new ideas. If you can convince me we'll see Ilyana Rasputin, Doug Ramsey, Danielle Moonstar, Deadpool, or Cable in the next eight years in the Marvel universe without delaying Dr. Strange 2, Black Panther 2, Captain Marvel 2, or the Inhumans then I will agree there's no downside. Until then, I'm skeptical.