Looking at the number of bands who are in the approximate area but NOT billed for Glastonbury, it feels like there are a LOT of acts (at all levels, across all days) who might be "on the bench" in case of Covid cancellations.
Harry Styles, Kings Of Leon, Green Day, Weezer, The War On Drugs, Dua Lipa, Rag N Bone Man, Jack White, Paolo Nutini, Pixies, Lewis Capaldi, Beck, Alt-J, Michael Kiwanuka, Jamie XX, Keane, Royal Blood, Maneskin, Pearl Jam - all playing nearby in the days around the festival, but NOT playing Glastonbury.
This doesn't even include some of the big "secret set" rumours like Arcade Fire, The Smile, Chemical Brothers - who presumably could turn into a "billed" act if needed. Plus there are big acts in early or "smaller than they could carry off" slots (Blossoms, Libertines), who could move around.
I suspect there are a lot of acts they could call on in reserve, if they had late cancellations - more than usual at this time of year!