IMO this is not a universal truth - I’m sure some people need that level of disconnection, but I don't find I'm one of them. I generally like my job, and don't find that forcing myself to disconnect does me any particular mental good. But other people report needing that separation, and that's fine! I don't think there needs to be a one-size-fits-all answer here.
I do agree with your bus factor argument though.
I think we believe ourselves to be more irreplaceable than we are. And if you really think you are irreplaceable then the problem is not going on vacation but being irreplaceable. Because then if something were to happen to you they are screwed.