What interest group besides unions had any interest in establishing a forty-hour work week?

I don't know an accurate answer to your question, but lets presume unions did it all.

I the GPs point is that having the power to change culture over the course of decades does not guarantee the ability to fix a single problem for a single individual.

Well, if the unions managed to secure reforms such that that person's problem would not occur then they'd solve it. It certainly seems more likely to help more people than doing everything piecemeal and on an individual basis.

A strong enough union can force all kinds of things, for better or worse. Whatever a company may gain by harassing a few ex-employees is nothing compared to the damage done by your entire workforce walking off the job.