Yeah I agree it would be a huge moment and a massive 'get' for the festival. I imagine the BBC viewing figures would be off the charts. Particularly now they've started broadcasting the headliners internationally (and last year could have been a dry run for her this year).
I do think regardless of success, her music is a lot more marmite than someone like Elton though, so would not necessarily pack out the field to the same extent. Or at least it might do initially and then a lot of people would find themselves leaving pretty quickly.
There would definitely be people queuing up at the pyramid all weekend to get a good spot though!
Massive Attack would easily headline Other Stage surely. They headline or sub every festival they play at worldwide - that includes in 2024 and 2025. If they're going to get a demotion at any festival, it's not going to be on home turf (or close enough). Was at the show in Bristol earlier this year and I thought it was incredible, despite the shite weather.