Given we know the festival apparently went for originally I'll give it Green Day. Falling in Reverse have also sold that ball park too but they aren't being used as one of the headliners. If I remember rightly, Sleep Token were the second replacement after Muse said no after Korn already replaced MCR. FIR even have the two London venues too. I'm not saying they shouldn't headline but they definitely weren't first choice and not more a seller than FIR for me. ST playing in the UK before edges them.
Because of this above. If and when she returns she'll sell an absolute wad of tickets that will be she won't need to hit Glastonbury yet. She's only 34, got years ahead of her to be able to do it is she wants.