> Looking back, do you think the team would have been better off hiring someone who came close enough, and supporting them to learn on the job?

More specialized.

If we wanted to train someone, we'd start with an internal candidate who was familiar with the other parts of the job and then train them on this one thing.

Hiring an outsider who doesn't know the subject matter and then teaching them is less efficient and more risky. It was better to have someone in the team learn the new subject as an incremental step and then backfill the simpler work they were doing.

Some academic departments do this... put a job ad every year in case there's a superstar.