As Russo hinted in his comments, Secret Wars' multiverse element would mean that Marvel Studios could dump in a load of new characters with a solid explanation that wouldn't compromise too much on the existing MCU, which will already be in disarray after the Infinity Saga anyway.
So, the biggest value in a Secret Wars movie would be the possibility to explain what will prove to be an otherwise inexplicable change to the very make-up of the MCU. Introducing the Fantastic Four at this point wouldn't be too much of an issue, as they could simply have an origin story, but for the past decade, there has been ZERO mention of mutantkind existing in this universe.
Marvel need an explanation for them when they do arrive, and it can't simply be a catch-all retcon. That would be deeply unsatisfying, to say the least, but having them come in as residents of another strand of the multiverse (which has been destroyed.