That sucks. Hope GNR could pull off the show on Monday. But unlikely.
When was the last time this happened? Rio 2010?
It's a festival. Just moving the show to another day is not easy. Bands have time slots when they play.
All the bands who play on that same stage basically have their gear set up for their shows that day. Then once the show ends, the gear is moved out before the next day's bands bring theirs in.
So basically as soon as the GN'R gear was off the stage, the Nine Inch Nails gear was loaded in.
Yes. A show got postponed in Rio de Janeiro in 2010 due to the stage partially collapsing before doors were opened. That wasn't a festival so it was possible to find an available date later in the same tour.
/jarmo