If the potara fusion being permanent was an undisputed fact, why arent Vegeta and Goku still fused? The simple answer is, because they didn't want them to stay fused forever. I think Vegeta and Goku staying as Vegito and becoming gods would be awesome, but, is that likely to happen? If that isn't what's going to happen, some way they are going to split. Maybe Beerus and Whis will find some way after the battle, it's certainly possible. But I wouldn't put it past them for them to split up because the power is too large to control, or the earrings break, or some other bullshit. In either case this is an anime/manga, nothing is set in stone