Sorry, misread you — but you can substitute ‘monopsony’ into my comment and I think it still holds.

This is a ‘no true Scotsman’ so I don't think I can really respond to it directly. But I'll point out that my claim is not that some contracts bargaining for safety of life and limb are a form of duress but that all inherently are (to some extent). Especially when the other party's BATNA is ‘no guarantee of safety’.