Airlines are insured for these eventualities
Actually they are not. Policies to protect airlines from eventualities, that nobody expects, are not available. Let's not forget that insurance companies too are expected to make money - meaning that they have to collect more than they pay out. Airlines couldn't afford the premiums.
Clearly a unprecedented shutdown of airspace is probably not foreseen in these rules, but are we suggesting that passengers should be abandoned?
I'm certainly not suggesting passengers be abandoned, but why should it be one part of the delivery chain that is responsible. There are no obligations on hotels, nor on surface transport, nor on restaurants - just the airlines? And they can't insure against losses.