Answered elsewhere: If we're investing in someone's training we'll promote someone from within who is already familiar with the product and then backfill their simpler work.

How do you know if someone is 80-90% there without having the job posting for the profile up, and interviewing candidates who come along?

That still takes a long time if random Senior Engineer X who's looking on LinkedIn is only 10% of the way there for what you'd need for a very specialized role.