1. These kinds of contracts don't come with provisions for compensation for every foreseeable contingency.
2. You can sue anyone for anything in America. But it's going to cost you.
3. You can also make future purchases conditional on how these contingencies were handled in the past.
So, it's likely that a lot of negotiation between Southwest and Boeing will happen. Possibly even some arbitration. If the two firms ever end up in court over this, that would indicate a colossal failure in all the processes that lead up to that moment. (As litigation is incredibly expensive.)
"crappy old airframe"
It's actually an incredible good design with a proven history. What exactly makes you think its such a "crappy" design?
I think you'd be hard pressed to find anyone familiar with aviation agree that the 737 is a poorly designed airframe...
If you think it's so great, then why don't you explain why you can't put today's large high-bypass turbofan engines on it in the normal position without them hitting the runway? How exactly is that a "great design"? It may have been a great design in 1967, but it isn't now, just like a 1967 Mustang chassis may have been OK then, but is totally inadequate now.