It isn't so much
IF they will fall off as
WHEN ...
A few times I have been wandering about on our local hills and I have been hit by sudden gusts of wind which almost knocked me off my feet. If that happened to those lads, they would be history!
I don't criticise people for taking 'sensible' calculated risks - downhill mountain biking, rock climbing, extreme caving, sky diving, and so on, but the risks in this case are way beyond 'sensible'. Their family and friends would have to suffer the consequences of their lunacy, and there would also be a grave risk to anybody who happened to be on the ground below them ...