Another random thought on workers and bargaining power: one of the reasons that I think that progressives assume that workers as a class have less bargaining power than employers is that workers with bargaining power generally use it to extract high salaries, whereupon they pass out of the class "worker" and into the class "wealthy".

Progressives don't simply want workers, as a group, to have as much bargaining power as employers; they also want that power to be distributed much more equally within the group. In that reading, one might argue that some significant portion of union redistribution is not from "employers" to "workers" but from "workers with bargaining power" to "workers with less highly demanded skill sets". Though I imagine that progressives would dispute that view.