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.

While Galactus did want to destroy Earth, it wasn't out of malevolence. It was simply because he felt he had to do it to survive. While Galactus is unimaginably powerful, he's not invulnerable.

“Forced to balance their roles as heroes with the strength of their family bond, they must defend Earth from a ravenous space god called Galactus and his enigmatic Herald, Silver Surfe ...