Is there a scenario in which all the major airlines collapse?
however, as long as they can cut expenses by laying off staff with no consequences, they're probably safe. previous tough times for airlines haven't been in quite the same situation where they can just cut expenses by cutting flights. i'd be more worried about the airports than the airlines - at some point, they're going to have a big backlog of fixed non-staff expenses that are no longer being covered by ramp fees.
IIRC American has some serious debt issues but Delta and United are sufficiently solvent that they'd get bailed out.