Again, I disagree.

Any reasonably complex codebase I have worked in has at least parts that only a few people understood. Even if n=3, you could still very easily be blocked when trying to find someone in the know...

Right, so an organizational and architectural problem. It's even got a specific term: Bus factor.

https://en.wikipedia.org/wiki/Bus_factor

You have a problem. Disagreeing with me won't fix it. It's a common problem, but also a fixable one.