The historical trend is for workweeks to steadily become shorter and shorter. But that trend has been interrupted and for some reason we've become stuck on the concept that a workweek is supposed to be 40 hours.
I'm not saying that capitalism is the problem, but our rigid adherance to certain extreme forms of free market economics...
Correct, there's nothing inherent in capitalism that says workweeks should be 40 hours.
One solution would be for the Department of Labor to index the length of the workweek, just as tax brackets are indexed to inflation to prevent bracket creep.
A better solution would not involve government at all. Salaries are just numbers that are freely negotiated between employers and individual employees, and the length of the workweek should be handled in the exact same way. (The problem, you see, is that it doesn't occur to most people that this particular degree of freedom should be one of the things that defines a truly free market.)
Employers always strive to attract quality employees, and they would gain a powerful edge if their initial offer to applicants could include, say, a 28-hour workweek. 40 hours was a rather arbitrary length when it was instituted, and it's even more arbitrary today.