Boeing can walk at any time. NASA cannot. Boeing has been paid for what they've delivered (plus some nice bribes to keep trying) and not for what they have not delivered so they an walk today if they want. NASA cannot walk though and would not.
Has anyone done the analysis on looking at the checks NASA is writing compared to the total contract price to know how front-loaded or how much money is still remaining in the contract? That seems like the easiest way to figure out whether its worth it for them to continue or not.
No, this doesn't seem the case here. The NASA's intent clearly was to get a spacecraft capable of flying to ISS. Where's that?
Boeing can walk away, but not without repercussions.