Except that 9 months took place on a space station. There were regular cargo missions to the station. And they could have been brought back at any nearly any point if necessary. Other astronauts literally went up and came back from the Station in that 9 months.

The timeframe being so long was almost entirely about the Starliner itself and what they were going to do with a known defective and potentially unusable spacecraft, where the only trained pilots were those astronauts, not anything with the astronauts themselves.

If the station wasn't an option for whatever reason (despite it literally being part of the planned mission), then other contingencies would have been available or at least planned already. This wasn't an Apollo 13 situation where not making it back was a serious concern.