At least in a startup environment, the bus factor risk isn't even on the same order of magnitude as much more tangible stuff like the client walking away. Doubling your labor costs to mitigate a risk that doesn't make the top 3 is just bad business. Sometimes it's not even an option.
You make a deal with your #1s that they will get the fun projects, the paid vacations when the current crisis is over, the completely flexible hours and full autonomy. Anything they want within reason, as long as shit gets done.
It's not for everyone, but I can see where GP is coming from.