If Kevin Feige and Bob Iger are to be believed, Avengers: Infinity War and its so-far unnamed sequel aren't just an ending (to the first decade of the MCU), but are also a crossroads for the franchise. While the movies may see the end of the first wave of Marvel characters (or some of them anyway) they will also serve as a promotion for some more new characters to the top of the Marvel table and a means to expand the MCU into new and exciting worlds.
Already, we know that the plan is to visit new parts of the Marvel universe, with James Gunn charged with overseeing more cosmic properties and there's also been talk of different time periods coming into focus too - in the vein of Captain Marvel's 1990s setting. None of that was particularly surprising, but then the Disney/Fox deal came out of nowhere (at least from a layman's point of view) and threw an almighty spanner in the works.
Having that many new characters to add to an established franchise timeline is a nice problem to have, but it's definitely a problem - certainly in terms of the logistics of their introduction. But there's an intriguing answer out there that has just reared its head thanks to one of the minds behind Infinity War.
Comments made by Joe Russo ahead of the release of the ensemble drove some speculation that he wants to make a Secret Wars movie as a means to bring in the formerly Fox-owned Marvel characters. And frankly, there's a lot to suggest that that would be the PERFECT way to address Phase 4's agenda for difference...
While some may have interpreted Russo's words to mean the 1984-85 Secret Wars mini-series, it's far more likely that he was referring to Jonathan Hickman's 2015 "sequel" in which a multiverse ending event brings together surviving elements of several alternate realities (via Doctor Doom basically becoming a God and fusing them together as a third Battleworld).
Eventually, the catastrophe is reversed and everything goes back to normal, but some of the characters from alternate realities - including the Ultimates dimension - ended up choosing to jump over to the mainline Marvel Comics universe, opening up new story possibilities for the publishers. In theory, it was a lucrative decision and a no-brainer.
That sentence there is exactly why Marvel Studios need to do it, but that's not the only reason...