People who think the labor movement was the creation of heroic union organizers have a problem to explain

This is a standard formulation that implies that you disagree with the statement "the labor movement was the creation of heroic union organizers."

That isn't the statement with which you want to disagree. You want to disagree with the person who thinks that we could return to the old days, if we were only more heroic.

Try something like this:

Some people think that the reason for the loss of the high-paying union job is a lack of moral courage in the modern age. Our ancestors were giants, and if we were more like them we could return to the golden age.

In fact the truth is simpler than the "fallen civilization" explanation . . .