If they're regularly losing out on major artists because of production limitations, or if they're just monitoring the kind of stages other festivals and major concerts are able to come up with, Glasto could do with seriously upgrading their ambitions when it comes to the Pyramid stage, at the very least (probably Other also). I think, with the benefit of complete ignorance about anything remotely practical and relevant, they could pretty easily do this around the current permanent structure (though they should probably upgrade this also at some point), and obviously while retaining their shape-based idiosyncrasy.
Madonna lived in the UK for absolutely ages and I think is still largely London-based at the moment. I'm sure she'll be well aware.
But it wouldn't need to be either / or. Lots of acts do both - Glastonbury for the hype and BST for the pay.