Of course they are, but to different degrees of pain.When you think about it, everyone is replaceable. You can be a lawyer, you are replaceable - there is always going to be someone else out there with that degree who can take over your job.
Fast food worker walks off the job you grab someone with a pulse and can have them mostly up to speed in one shift. That isn't the case with many professional jobs.
Takes at least 5 three hour shifts to get someone up to speed. Not one.
Would take the same amount of time to replace, say, a secretary, for example.