The markup doesn't necessarily make sense, unconditionally, because there's also the your own adaptations to the company to consider. If their environment makes you reasonably comfortable, that's a risk mitigation versus other jobs that's worth something to most people.

Also, few people are truly indispensable. In all my time working, I've never seen a company fall apart from the loss of one important person. In fact, it usually takes a shorter time to cope than anyone anticipates.

I do agree with you, In the sense that as an employee, you're in great shape if you're seen at being essential, to some degree; I just don't think you're capturing the entire market dynamic.