Note that while productivity has been decreasing, wages for the middle incomes have been stagnant, and wages for the lower incomes have been decreasing, allowing for inflation.
From "My Life and Work : An Autobiography of Henry Ford" Chapter 8 : Wages
I believe in the first place that, all other considerations aside, our own sales depend in a measure upon the wages we pay. If we can distribute high wages, then that money is going to be spent and it will serve to make storekeepers and distributors and manufacturers and workers in other lines more prosperous and their prosperity will be relected in our sales. Country-wide high wages spell country-wide prosperity, provided, however, the higher wages are paid for higher production.