It might be immoral, but surely keeping they planes will be legal by some means, executive fiat or whatever.

Is there international treaties covering such things?

Mind, I'm from the UK our government just ignore international law when they like and only to please their own ends, not even in times of crisis, so can't see something as simple as breaking a treaty will cause any change here.

Exactly. It'll be a slow and steady decline in terms of safety, pilot training and so on.