Fortunately, we developed systems of writing, so that the limit to this relearning is higher now than a few millennia ago. (Society does the ultimate “rehiring” continually.)
I think that far too many companies (including my own) rely much too heavily on “things that are in people’s heads” and don’t spend enough time and money on making the company knowledge effectively outlast any individual employee, making it easier to put the essential knowledge into new people’s heads.
Society doesn’t rediscover pi, e, I, and calculus from first principles every half-century, but rather we’ve instituted that knowledge into books and mechanisms of teaching.