Isn't that a slippery slope? Even if you worked a "proper" 40-hour week you could always ask yourself why don't you devote 41 (or 42, 43, and so on) hours per week for work. One could argue that your productivity plummets as you work more hours, but in absolute terms you're still getting more work done.
Also, consider this: instead of having two people work 20 hours a week we're having one person working 40. Shouldn't that person feel guilty about taking the opportunity to contribute and being valued from somebody else?
As for creating contribution through work - are we talking about salaried work? What about volunteer work then? If by work we mean anything that adds value whether we get paid for it or not, it's not always so obvious. The lines between enjoyable hobby and work are blurry at times and an author who writes for fun could end up creating a piece which inspires millions.
In the quest for maximizing collective productivity of the humanity we could start introducing teenagers into workforce as soon as possible and scrap the retirement age. Maybe we could even leave out one or two public holidays and add couple of work hours on Saturdays. Some would complain, sure, but overall we would become more productive.
Personally, I don't think that's the way to go. 40-hour work week is a relic of the 19th century and time is a poor indicator of productive contribution.