I would argue that the linchpin of that situation is an overly generous bankruptcy system that allows failed airlines to wipe off their debt, secure new capital and go about destroying it as before. If more failed airlines had been allowed to disappear à la Eastern, we might have more airlines, more competition, and smaller networks today.
I completely agree that U.S. bankruptcy laws have had consequences for this industry that are arguably unintended. Now the whole debate about the ultimate efficacy of those laws, and their original intent, is really a separate discussion altogether that has implications far beyond just the airline industry.
Nonetheless, I don't agree that tighter bankruptcy laws - like those more common in the rest of the world - would likely have led to the outcomes above. Ultimately, what the market has been naturally driving the airline industry towards since deregulation in 1978 is consolidation in order to create a balanced, economically sustainable industry structure. One way or another, that outcome was going to occur - over the long-run, economics required
it. The only question was how and when.
Had bankruptcy laws been such that insolvent airlines were more likely to go out of business as opposed to restructure, we would have ended up with fewer, larger competitors sooner. I would personally submit that, in such a situation where bankruptcy essentially meant liquidation, certainly USAirways and quite possibly also United both would have ceased to exist in the period following 9/11. Both airlines' finances were particularly precarious during that period. And how would the industry have responded? The surviving airlines would have swooped in to pick up the pieces and fill the void, and been better off financially as a result. But that didn't happen. Instead, financially failed carriers limped along, using bankruptcy one or multiple times to restructure their finances. And then they merged with each other.
Personally, I still believe that the outcome would have been approximately the same either way - consolidation was inevitable, whether through mergers or liquidations.