Starliner not planned as a lifeboat, no one else has suits or training for it. Regardless it only has a 90 day in orbit clearance and is now over 66 days so has to return.
ISS currently has 2 more on board than planned and has to reduce total.
Plan is to launch the next Dragon with only 2 of the planned 4 crew, with 2 of the current crew extending their tour until early 2025.
The Dragon will return of the planned 4 crew rotation plus the Starliner crew. The assumption is the delay will allow two necessary requirements.
First, production of suitable Dragon suits to be sent up on the ship for their use on feature. Second the upload of a software update to the Starliner to allow an automated unmanned recovery freeing up the docking port for the Dragon.
If that happens the major decision for Boeing will be whether to constitute with the NASA contract.
Starliner would undoubtedly need a further redesign and qualification flight. With the remaining life of the ISS and available slots there would be a maximum of a further 3 manned flights available for Starliner, each costing Boeing $xxM in losses and possibility of further embarrassment.
Probability is that, if they come bck using SpaceX, either Boeing or NASA will pull out of the contract.