Very unlikely, SW and AS are too big to fail, the way the industry is currently structured. They might or might not be too big to preemptively merge with each other depending regulatory climate.

If the big three suppress SW/AS over many years, they might hypothetically shrink to the point where they become small enough to fail. But in the near/mid term if they struggled financially they would be allowed to be acquired.

Even B6 liquidating would be disruptive in the NE, kind of doubt they'd just be allowed to die either.