Metallica headlining Glastonbury? What a disaster in the making.
There will be a contingent there that will want it. There will be many, many others who will not submit themselves to the assault that is a Metallica show. This is not like being a rock and roll fan and wandering over to see Jay-Z play. You don't just wander over in your Wellies and watch the band perform. When you stand where Metallica are playing, you are going to be relentlessly pounded by sounds. And if people want that, okay.
My guess is that the Metallica crowd will roll in for the show and leave when it's done and that everyone will go see Jake Bugg.