Just to be clear on this again, they aren't "stuck" because they're in danger, the stay keeps getting extended so engineers can gather as much information as possible about the leaks before they return. Starliner is still fully capable of reentry, but this is Boeing's last chance to collect data that can be used to fix these issues in the future. The leak problem is in Starliner's service module, which detaches from the capsule before reentry and burns up in the atmosphere, so there won't be anything left to study after the astronauts return.