Most people don't work jobs they care about. In an ideal world how people choose to spend their time would be entirely separate from notions of productivity or wages. But the fact is that we operate under a terrible economic system and people are overworked by largely meaningless jobs. In the context of society as it stands today, not only are people overworked, but most people could work less without their respective employers losing money. Because modern businesses operate solely off the principal of maximizing profit, I think it's poignant to point out that it's in their benefit to reduce the work week for their employees.
You can't apply this notion to people who spend their time in meaningful ways. You couldn't say that Picasso would be just as effective with a shorter work week. If people lived reasonable lives and did genuine work, the whole idea of comparing length of work weeks with effectiveness of work would be meaningless. You point out that when people are worn out with a certain activity, they could benefit by just moving on to another. That's true. Reasonably speaking, people shouldn't do anything for even a minute longer than they feel like. But the corporate world does not offer that kind of flexibility. The corporate world is rigid, and people are forced to operate in unnatural ways. Repetition and narrow focus are the bread and butter of this system, meaning you perform a single task over and over for the duration of your work day and that's it. This is the context with which you must examine the idea that "shorter work weeks are just as effective" because this is how most jobs work. I think the reason for pointing out that "shorter work weeks are just as effective" is really to use the ruthless, disconnected, corporate logic of the modern business world against itself.
Now, let me just dispel a myth you stated. You believe that "Given two equally talented people, the one that pursues a goal obsessively, for well over 40 hours a week, is going to achieve more." This again is modern corporate mentality. Work harder, put in more effort, and you will achieve more. The real world does not operate on these principals. Only menial tasks can be translated into the notion of more effort = higher yields. Lets take the example of Picasso again. Do you think the more time he spent painting in a given week the better his paintings were? Or take Newton. Do you think if he sat down and forced himself to ponder the physical laws of the universe for ten more hours each week he would have made more discoveries? This is a simplistic and completely insubstantial view on the world.
If you examine the lives of truly successful people (by which I mean people like Newton or Picasso or Tchaikovsky, not financially successful like Mark Cuban or Bill Gates) then you realize the only "rule" you can extrapolate from their lives is that people operate best when they do so on their own terms. Not when they work for longer hours or with more perceived "effort", but when they work exactly as they please.
But anyway, that's getting into another topic. I just wanted to point out that the idea that shorter work weeks are just as effective is completely valid when applied to the current and deeply flawed structure of modern work environments. It's not something which can be applied to ideal scenarios which offer flexibility, and frankly that's irrelevant because most people aren't fortunate enough to have that. If we do get to the point where people can work under truly ideal circumstances, the entire notion of productivity or effectiveness will be meaningless. These are concepts which only hinder good work.