I wonder if the astronauts themselves get some say in this. What if they decide, since it is their lives, that they're not getting into the starliner, even if NASA decides the risk is acceptable?
However, I imagine that part of becoming an astronaut means that you really have to get comfortable with trusting others to make critical, ultimately life-affecting decisions on your behalf all the time. So perhaps their mindset is more of "we trust that all the smart people on the ground are doing their best to make the safest decision for us, and we'll go with what you say".
If I were one of them stuck up there, though, I'd probably want to get on a video call with the Boeing engineers and look them in the eyes, show them pictures of my family, and ask if they are confident their vehicle will bring me home safely.