I was about to say something similar but then checked out Sampha. He’d only got two albums and headlined MS last year, so I suppose it’s possible (hopefully not probable) that Kneecap could do it. Much more likely for the tent or Thursday imo
I'd say there's almost the opposite problem.
Namely that some of their bookings were made with the expectation that the film would catch on and perform a lot better than it did, and so that the band would be a bigger deal by now than they actually are.
So as a result they've ended up getting some bookings bigger than they really should do - if they're headlining Green Man then that would be a prime example. It'd be because the Festival made a gamble that didn't pay off.