Look, I'm not arguing for forbidding people from doing work they enjoy, the idea is rather to put slightly different incentives in place.

You might not be. The article linked, however, is doing exactly that:

Achieving shorter working hours. Conditions necessary for successfully reducing paid working hours include reducing hours gradually over a number of years in line with annual wage increments; changing the way work is managed to discourage overtime; providing active training to combat skills shortages and to help long-term unemployed return to the labour force; managing employers’ costs to reward rather than penalise taking on extra staff; ensuring more stable and equal distribution of earnings; introducing regulations to standardise hours that also promote flexible arrangements to suit employees, such as job sharing, extended care leave and sabbaticals; and offering more and better protection for the self-employed against the effects of low pay, long hours, and job insecurity.

In a nutshell, the argument is "disincentivize people from working hard." And again, it's been tried. It should also be pointed out that the actual study arrived at "21 hours" arbitrarily:

21 hours is close to the average that people of working age in Britain spend in paid work and just a little more than the average spent in unpaid work.

If they used similar methodology in the US, their paper would be titled "the 46 hour work week."

It seems much better for people bored to death at work for it to be socially acceptable to work less, especially when half the population would be working full time and the other half not having a job at all.

Except that's not the problem we're having. Tim Ferriss' "4 hour work week" has been through a dozen printings. Everyone wants to be able to work less. The trick is it's really hard to get ahead if you do. The study listed above (and "study" is generous) is arguing that people should stop trying to get ahead.

I watched your TED talk, by the way. You recognize that it's talking about unskilled labor, right? And that markets such as this already exist around Home Depot? Amazon's Mechanical Turk will also employ your idle hands for pennies whenever you want a little scratch on the side. However, when you get into skilled labor everything changes. I can call up a babysitter to be here in 45 minutes. It will cost me a $75 travel fee and $18 an hour with a 3 hour minimum. Hell, as a IATSE Y-1 sound mixer I've gotten calls at 1am to be mixing at 5am the next day. My posted rate is $57 an hour. I often work 8 hours or less in a week. Sometimes, however, I'll work 26 hours in a day. I have all the protections and such that this article would like the rest of the world to have, and my working conditions are so bad there's a documentary about it.

I just think we should curtail over-consumption and actually let people enjoy the fruits of the record high productivity that automation brings.

So say we all. The trick is finding a practical way to get there. My argument, which I hope I've expounded and explained adequately, is that a '21 hour work week' is a stupid and useless way to do it.

A lot of people work way too much currently, grinding themselves down instead of investing in themselves and leading sustainable lives, by spending more time on learning and growing as human beings.

Sure. But if we're going to fight to change things, I'd rather have 6 weeks of paid vacation than a 21 hour work week. How 'bout paid maternity leave? Maybe government healthcare? A livable minimum wage? There are all sorts of things that level the playing field in a proven and effective way that could be instituted with a lot less drama and a lot more success than making everyone suddenly go part time.