Damn you know they have real doubt about Starliner with this decision. People were saying Nasa was just being super careful and the vehicle was perfectly fine like Boeing said. But with the astronauts being forced to use Crew Dragon, they can't wear their space suits. Because the suits they use on Starliner aren't compatible with all the SpaceX stuff. This can be a big risk, if there is a problem with the vehicle, the astronauts have no protection. But Nasa must have decided that was a smaller risk over using the Starliner. That says a lot.
Can't they just send up extra suits with the dragon capsule? Badly fitted suits are probably better than none, it's not like they are piloting it down.
The last thing NASA wants to do is to loose astronauts during a mission. This would just be compounded by the fact it is an election year.
If returning with a proven Dragon capsule is even a few percentage points safer than returning with Starliner that has thruster issues, they are going to do it.
Boeing royally screwed up the entire Dreamliner program as well as Artemis production. With Sierra’s Dreamchaser potentially coming online as third alternative for manned space flight. I doubt NASA has fewer issues with letting Boeing go at this point.