Nobody except newcomers to the codebase look at documents on code structure, and of course no one is fired for not producing it/producing a crappy version of it.

When the newcomer can't get anything out of that doc, if it exist, he/she just need to ask the creator, who will explain very nicely how everything is done, and that newcomer become a code guru in that codebase, and the cycle continues.

Until you layoff 50% of the team and now suddenly nobody knows how 50% of the stuff work.

Very frequently in corporate environments, at least in investment banks I have known and heard of many people earning four figures a day pretty much purely because they've been allowed to carve their own niche that becomes indispensable.

Ironically, the worse they are (think 5000 lines of Perl with two functions) then the less viable it is to replace them.