I do agree with you both, but we need a wider view of what documentation means. I'm spending time this days reading the OpenBSD code and while the latter is really well written, I spent even more time reading the mailing list archives, the revision notes, the BSD4.4 book, some OS textbooks (Three Easy Pieces is great BTW), and various datasheets (Intel, USB,...). I would have a much easier time if I could find a dev to be my mentor and walk me through it.

All of this to say, tribal knowledge is valuable and rarely well captured. Even experts organize conferences and lectures instead of just sending each other papers. Written words is better than nothing, but knowledge is better transferred from the person that has it. If you lose that person, the only way forward is to train another one to replace it (either intentionally or not).