Back on April 4th when I first broke the story that Marvel is doing The Eternals, one of the things people used to try to debunk the movie is the pending Fox merger Disney is in progress with. Except the timing of that completion is why we’re still going to see The Eternals before the X-Men or Fantastic Four are able to join the MCU.
Movies take time to prepare, and Marvel Studios cannot legally work on any of the Fox characters yet. That even means nothing as “innocent” as concept art. Sure, Kevin Feige can daydream about how they’ll use them when they can, but until the Disney merger finishes sometime next summer; no work can start on any of the Fox characters. Yet Marvel still has to release a slate of movies every year.
The first two 2019 movies are already nearing completion, and Spider-Man 2 will start filming soon. But Marvel still needs to plan for 2020 and 2021 as it’ll take time to get those movies going and they can’t sit around hoping for the merger to complete.
That’s why James Gunn is currently writing Guardians of the Galaxy Vol. 3, writers are working on Black Widow as the director search continues, and writers were just hired for The Eternals.
Sersi, Ikaris, and the rest aren’t expected to make their big screen debut until 2021, however the rumor is that they’ll be introduced in the MCU in some way prior to then.
That’s why stuff on The Eternals is progressing pretty quickly and Marvel isn’t going to shelve it for some movies they can’t even begin to work on until next year sometime.