> Boeing doesn't know what's failing with the thrusters. They haven't been able to give NASA a root cause so even though they got 4 of the 5 anomalous thrusters working again, who knows what could fail on the return flight.
Apparently, the root cause is thermal overheating due to a design flaw in the thruster enclosure ("doghouse"), where the design provides insufficient cooling. [0] The problematic thrusters are all in the same area, which is likely the worst affected by the overheating. Boeing can't explain exactly why this particular thruster within that area failed permanently and the other four only failed intermittently, but when you are pushing a system to its thermal limits, it is expected that the outcome will be less than completely deterministic. And of course, Boeing wouldn't be eager to give the root cause as "our design is flawed and we didn't do adequate pre-flight testing to pick up on it, and our thermal analysis was flawed too". Much better to say "we still don't know".
> Boeing should bail and NASA will grumble at the sunk cost of their failed pursuit of redundancy but everyone will be better off.
Boeing can't bail. If they break the contract, it will be a black mark on their federal contracting record, which will harm their ability to win new contracts – not just with NASA, but more importantly with the Pentagon, which is where the real money is for Boeing.
That said, if Boeing really wants out, they will likely lobby to have NASA terminate the contract "for the convenience of the government" – so officially it is NASA who is cancelling it, with the result that Boeing's federal contracting record remains clean on paper.
[0] https://www.space.com/boeing-starliner-iss-hot-fire-reentry-...