I don't think that's a complete explanation. I would say it's more along the lines of "If you start with somebody working a three-day week, it's much easier to employ them for another two days, than to hire a new person to work two days because that requires creating a whole new business relationship." Then both corporations and governments, I think, tend to be as inefficient as they can possibly get away with without dying, or maybe a little more inefficient than that. Work expands to fill the time available...
I would have to sit down and write this out if I really wanted to think it through, but roughly I think that there are forces which tend to make people employed for a full workweek, everyone want to be employed, and society to become as inefficient as it can get away with. Combine these factors and it's why increasing productivity doesn't increase leisure.
The full work week makes sense, depending on what sort of job you're talking about. Is it a job where a certain number of staff have to be working at a given time but it doesn't really matter who, i.e. my job at the pool, etc, or is it a job where a certain amount of work has to get done and it's simpler for one person to do a set of tasks because sharing the tasks between brains is complicated, i.e. my job at the research institute? For the former, it doesn't really matter whether you have 20 staff working 40 hours a week or 40 staff working 20 hours a we...
From Mike Darwn's Chronopause, an essay titled "Would You Like Another Plate of This?", discussing people's attitudes to life:
Conclusion, graphs, and references in article. As usual, I recommend reading Chronopause.com as Darwin has many good articles; to quickly link a few: