As I found a verdict on one forum: if the seller of a service or product knows in advance that he cannot offer it, then it is a pure scam.
Airlines know very precisely how much staff is needed to serve x amount of passengers. Airports know the same.
All this in its nature excludes any extraordinary circumstance. And the whole mess in European aviation today is 100% the fault of the innocent greed of the airlines.
Consequently, no traveler needs to sympathize with them or try to understand them. On the contrary -- everyone should demand their compensations. So that airlines will no longer be tempted to sell significantly more tickets than they can actually serve passengers. And if they sell, it would cost them.