The problem I have is that only people who "know" already tend to accept this insight. i.e. explaining it to those who make decisions doesn't work that well be

The problem I have is that only people who "know" already tend to accept this insight. i.e. explaining it to those who make decisions doesn't work that well because it doesn't fit their mental model very well - they don't understand that a large part of their asset is sitting in people's heads. That guy wants 5k more.....? Pay it. The cost of hiring someone new and training them up and relearning it all will be far higher. Don't force people back to the office, be relaxed about everything and keep the knowledge. At the same time make sure other people are learning it so you do have replacements. At the same time make sure you have proper tests so you can work with code you don't understand yet if someone leaves. At the same time try to gather all the correct and uptodate documentation and explanations somewhere.