Good point. I know this as the Mack Truck Theory. For the project I'm working on right now, there's a couple of incredibly valuable people that would cause a pretty significant issue if they disappeared.
It's all fun and lulz until it happens to you.
You'd be surprised, but that's not necessarily the case.
One of my friends was such a person in a shoe making company as a designer. Instead of giving her a raise, they fired her.
Cue them re-hiring her a month or two later after they found out the hard way that the less experience subordinate really couldn't handle the job of the two of them on their own.