It's really only a large risk if you don't compensate competitively.
Sports teams constantly "lose talent" because they refuse to pay for it. Suddenly they start losing games..
All this "knowledge lost" is often just talent and experience.
You can lead a horse to water, you can lead a junior to documentation.
I know the manager brigade will take this personally but talent and experience have always carried any workplace I've been a part of.
Compensating competitively might stop somebody from leaving for another job, but it won’t stop them from keeling over dead from a heart attack or getting hit by a car.
You need to plan in advance for people leaving, not just assume they will be there forever.
Or moving, or just wanting a different job. Money helps, but it doesn't solve everything.