Good question tbf. Both maybe?
I think there is being really popular then there is headlining Glastonbury because you're a good fit, and you are really popular.
I just don't think AG does? I know she did Coachella (which is now a celeb thing more than anything) however I really don't see her as a Glasto festival headliner. At least in recent years with Adele and Ed (solo headliner artists) there was this understanding from people who don't like them that they are at the top of their game and arguably should be there - I think AG will be seen as a bit of a shit booking by people who aren't keen on her, I feel people won't be as understanding as they were with Stormzy, Ed and Adele yet. I think that's kind of why they won't book her (especially alongside the 1975, who let's face it, aren't widely loved in the slightest).
I think you obviously have to be elite to headline.
Personally, I don't see her in the elite class of female solo Pop artists. Gaga, Beyonce, Swift, Adele etc.
I think this kinda feeds back into my first point, if any of them 4 headline, to joe public and the media, it makes sense and it's a solid booking. With AG, it's almost as if they are taking a punt (which doesn't make sense because she IS huge) to get her on. Yeah, last 2 albums were decent but was anything before that good? Not that great imo.
Let me just say tho, she is fucking huge. Of course I understand how big she is.