If there is a risk of a 10+ million dollar issue there is also some manager whose job is to overreact when they hear the announcement that someone wants to eliminate thing X, because they know that thing X is a useful part of the systems they are responsible for.

In a reasonable organization only very minor systems can be undocumented enough to fall through the cracks.

In an ideal world sure, but knowledge gets lost every time someone randomly quits, dies, retires etc. Stuff that’s been working fine for years is easy for a team to forget about.