An architect that doesn't leave documentation and share knowledge isn't doing their job and management needs to get ahead of this. What if this "key person" decides to find another job?
This is especially bad for software that are written under tight deadline - no matter how much document you write about it. And sadly, there's more software that are written like that than otherwise.
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.
Ironically, the worse they are (think 5000 lines of Perl with two functions) then the less viable it is to replace them.