Productivity does not simply mean "good work ethic". Technology is largely responsible for those productivity gains.
Even with stagnated wages, its not to the point where all of the money is being sucked up by the rich. I mean, look around you. The people like the article is describing, people who have "good jobs" and retirement plans but yet no savings - look at these younger people you know around you that are in the same sort of situation and look at the things they do. So much spending, not a lot of saving.
Prior to 2008, we were buying things left and right on cheap credit. Living way beyond our means. Spending far too much and not saving enough. Buying too much home, too much car, etc. than what we could really afford, living high on the hog so to speak when we really shouldn't have.
You can't simply blame all of the savings rate issues on stagnant wages, more wealth concentrated at the top.
The savings rate from 1995 onward dipped down from 6% to 2/3% by the early 00s, in the middle of an economic boom. Americans were simply spending the fuck out of their money. Then along comes 2008, and all of a sudden we return to reasonable personal savings rates. Although this has since declined with the upswing in the economy since...