I think it’s more expecting them to tour in some form so could turn up at BST if either decide against stadiums. Probably more aimed at Muse considering the high ticket prices and not so fast selling arena tour.
Used had one of the biggest crowds in the Wave last year. Not really my thing but enjoyed it and the crowd were bang up for it. Might be too much of a step up on the Main though.
Thing is it depends on what the issues are - building a bigger stage would likely bring the Pyramid up to the relatively standard 20x20 space for "on stage" production which does make it easier for touring setups. But if for example she wants big / complex extensions going out in the field (such as she had in Rio) that would remain just as awkward to achieve without massively disrupting the rest of the day or potentially even weekend.