Ultimately, I think large scale personal air travel (or mass transit in general) isn't feasible in all economic climates. By definition, it's high upfront costs and low margins. The only airlines who seem to always make it work are regional and shuttles between high-income cities (Alaska, JetBlue.) As for why don't we just nationalize them, I'd say we de facto have already. Airlines know bankruptcy-level downside is covered by the government.
It's not all bad though; there's an argument to be made that this system is pareto efficient. During bull times, you want private operators pushing for network expansions and price efficiencies, even if that means levering up. But these are a de facto utility, so having the government pinch their nose and cover the downside in a bear market is an efficient use of dollars. You could try and put in regulation to force them to reserve X months (years isn't realistic) of operation costs, but you can't get around the basic unit economics of the problem: planes are expensive, even if they're grounded and don't make money if they can't fly.