Through out this whole history of labor leaders calling for a shorter workday, somehow the union bosses always assumed that the paycheck would stay big while work hours got small. They never saw a problem w/ that and were always dismissive of concerns over production and business' labor costs.
Ignore reality and it bites.
I don't think they really gave a rat's ass about the paychecks. They were assuming the union dues would stay the same, and the companies would have to hire more workers, so there'd be more of them.