That is the exact problem with the 4 hour workday and other forms of part-time work. As I've heard many people describe part-time work : 40% of the pay for 80% of the work.
Until that changes, I would not expect part-time work to happen for anyone but really high up managers where such a trade may actually make sense.
If there are any problems about work, they are about making it more enjoyable, making good work more widely accessible and distributing the proceeds fairly.