What I don't understand is what your boss thought he would gain by the threat? Because if you quit, they'll lose weeks or months trying to find a replacement?

What I don't understand is what your boss thought he would gain by the threat? Because if you quit, they'll lose weeks or months trying to find a replacement? And if the environment is toxic and they'll repeatedly try to get people to do the job, they'll get absolutely nothing done in the long run?