It remains to be seen whether Galactus will destroy the Fantastic Four’s retro Earth in First Steps. However, an online theory indicates that Galactus will destroy the retro Earth by devouring it.

In that seminal 1966 issue, the Fantastic Four are visited by an all-knowing being called The Watcher, who warns the team that Galactus is on his way to consume Earth and that he’s tried to hide ...

That's why the most popular theory is they'll get warped into the MCU proper (or 'The Sacred Timeline' of Earth-616) at the end of the movie. They'll have Galactus to deal with before then ...