Fixed price contracts can be quite a problem in high risk projects.
The issue is that incentives absolutely aren't aligned. The moment Boeing thinks it has achieved a certain part of the contract but NASA does not you have a huge problem on your hand where one side will try to get work done for free and the other has absolutely zero interest as they are not getting paid.
If cost overruns are too great Boeing might also (depending on the contract) choose to withdraw, as the continuing development cost will be greater than any penalties from the contract.
As much as I like the idea of having multiple independent vehicles (and knowing the impact of having your only crew-capable launch system grounded when it happened in the shuttle era), Starliner has been in systems integration hell for years now, always just a couple short months away. Boeing withdrawing and NASA restarting the bidding process for that contract might benefit everyone.
I would exclude SpaceX from the bid, but Sierra Space is the obvious other option with their Dream Chaser. Apart from Sierra and ULA there is also everyone who bid on the lunar lander, meaning Dynetics and the "National Team" (Blue Origin, Northrop Grumman and Lockheed Martin)
The issue is that incentives absolutely aren't aligned. The moment Boeing thinks it has achieved a certain part of the contract but NASA does not you have a huge problem on your hand where one side will try to get work done for free and the other has absolutely zero interest as they are not getting paid.
If cost overruns are too great Boeing might also (depending on the contract) choose to withdraw, as the continuing development cost will be greater than any penalties from the contract.