It helps that in the comics X-men usually have their own thing going on so it feels much easier to separate them from the rest of the Universe. There's just a ton of characters, teams, lore, and story lines that you can make a whole separate cinematic universe with just that one property alone. I don't think FF has anywhere near the potential that X-men has as a franchise, but their concept, character, and lore would be a perfect fit for the MCU. FF returning to Marvel would be the best outcome for that property. As long as Fox keeps the trend we see with Logan and Deadpool going I'm perfectly fine with them keeping X-men, even if they occasionally make a bad film.