That is, if automation makes things twice as efficient, it means everybody can work half time and enjoy 20 more hours of leisure a week.
The above can be possible, but you may not get full time work even if you wanted. Not everyone can be retrained for software programming or robotics, and even if they were, you don't need millions and millions of people with such skillset.
That's why a growing acceptance or realization that some type of guaranteed allowance will have to be given to a majority when the jobs vanish. Many countries are already in that path - in US you have social security which kicks in after retirement, what if it kicks in early? (You need political will, and much greater acceptance to the truism "jobs are not there".) In India there is 'Right to Food', which can be the beginning of such programs.
Indian politicians and Chinese have may be a decade of "more growth for prosperity" philosophy to work its charm. US and other developed nations got no such space.
Of course, it's not actually true because people have an amazing capacity to come up with new products and services.
Here is the issue, the new products and services may not generate "mass employment". Amazon, Facebook and Google are great examples. The same with Tesla.
"Mass employment" happens with manufacturing or may be Walmart (not only employment at their stores, but building the stores to the driving the trucks), not with the new products or services you have in mind.