It’s absolutely true that reacquiring the film rights to the X-Men and other Marvel properties will be useful for Marvel.
Having said that, while the X-Men movies have always been reasonably successful, Deadpool notwithstanding, they’ve never been the super-blockbusters that many MCU movies turn out to be.
It gives them new ways to expand the MCU which will be exciting for fans, enticing them into the theater.
Fox also has some lingering rights to Star Wars from the studio’s days distributing the first two trilogies, and while Disney will certainly love to make that money themselves, considering how huge Star Wars is, the added revenue will probably be a drop in the bucket.