Boeing deserves the overwhelming majority of the blame here, not one of their many, many customers. Southwest did not want a plane so unsafe that it cannot fly. If their wish list desires were not all reasonable, then it's Boeing's responsibility to let them know it.
That is a different point. Southwest (they are not alone, though they are the obvious example) is not going to buy any plane from someone else because the 737 is so embedded in their company. They can take something else, and reserve the right if Boeing cannot deliver. However it is unlikely anything other than the 737 will be bought in the foreseeable future.
Now if someone built a plane with the same type certificate required with capacity from 25 to 300 (exact bound are of course negotiable) passengers and can deliver in quantity that would catch South west's attention and probably change their entire fleet. I suspect that isn't possible but...
