If automation and efficiency could bring most people to the point where they could hit this point of diminishing returns at 20 hours a week, I think we'd find more (though not all) people opting for that kind of life.
You're also making an assumption about which variable will be influenced by "automation and efficiency", as you put it, and how it will be affected. Throughout the human history, the technology has helped us overcome obstacles and "optimize" our lives. Where before you would need to invest significant effort to overcome an obstacle, now you have technology to do it for you. What's the consequence of that? The human effort that was previously needed to overcome that obstacle has become devalued. In other words, technology is more likely to make someone's work obsolete than to allow them to lay back and enjoy having a 20-hour work week. Simply put, whenever technology simplifies a part of your work, you either get more work or you're out of work.
It's not that I wouldn't like having a 20-hour work week. It's just that I don't think technology is going to solve that.