While I am in no way a devoted fan, personally I'd be very happy to see The 1975 headline -- never seen them live, enjoy a good number of their tracks and IMHO they're sure to put on a good show.
Equally, I was as far away from Pyramid on Saturday this year as I could possibly be as I am one of those Coldplay-unenjoyers -- I've just never got the appeal. Was quite happy for them to be headlining (again!) though, because I know that a lot of people do enjoy their music and from what I heard, a lot of fun was had. But there are so many other options at Glastonbury it really doesn't bother me who is headlining where and when -- personally I took the Coldplay slot as an opportunity to do something totally different on Saturday night and went to Arcadia to see Barry Can't Swim do a DJ set and had a bloody great time!
Anyway, I reckon The 1975 would be a great booking, would be perfect headline material and attract a very decent crowd.
It'll all depend on their next record. They are relentlessly touring, still, and that ends at the end of next summer. They've been prolific with writing, so I don't think it will be too long once they've had a break.
They turned down the sub-slot for 2025 because they believe they'll headline. I think they will too, but I'm bias. Very bias.
One of the headliners in 2027 or 2028, in my opinion, but time will tell.
I think these will be in thinking for 2028 on a next album to headline, 2027 will probably be too soon but if they took 26 out to write a new album who knows. They aren't a headline spot this year but I think Glastonbury tried to be clever and sneak them in as a sub on a year whereby sales show it's a bit borderline of being above a sub/but not headline. They don't return. Without being top spot. They aren't for me but for the guitar/band music industry it's great to have them as an option going forwards.