Chat was that they were offered Pyramid sub for 2025 and turned it down, if I recall.
They'll need a late 2026 / early 2027 record that matches or eclipses the commercial success of Romance to be on the level, likely with a winter arena tour proper around that time. They sell three nights at The O2, then we are talking.
I would also take them as a credible "young" and "homegrown" (for a given definition) headline option if they have bagged two mega-stars to top elsewhere - for argument's sake, let's say Rihanna and Lady Gaga.
There is another factor at work though, namely that I have as much chance of being approached to play Radio 1 Big Weekend as Sharon Van Etten does. I'm not sure she'd even be on 6 Music Festival's radar.