if 80% of knowledge is missing due to 80% of people being gone then the team who built it failed to document or automate themselves out of a job, meaning even other people on the team in the good ol' days would still have faced the same issues and merely had to chase down the original authors. that doesn't sound like it would be completely true. maybe 5-20% of un-documented knowledge walked out the door? completely rough guess. but even 0.5% of knowledge can sometimes be critical.