With flexible work schedules and environments becoming the norm, the change to the typical 5 day, 40 hour work week is the next to go. Working schedules of longer days, for example 4, 10 hour days are more typical. Families needing alternatives to child care and making their work schedules fit their children's schedules to avoid spending their extra income on paying others to watch their children. People are desiring a more balance to their lives between work and life- and the change to a 3 or 4 day work week is just the beginning.
I think that the five day work week is here to stay. Most business still need to be open five or more days a week to serve the public. They will still need workers to cover those hours and will not want to hire more staff to cover different days.
I do not think a 3 or 4 day week will ever become the norm. People have an instinct to earn more money. No matter how much money you paid people for a 3 or 4 day week, they would always want to work 5 days to earn more. People only need so much leisure time.
Periodically, discussions emerge touting the benefits of cutting the standard work week back to 3 or 4 days. Proponents argue that shortening the work week would increase productivity by giving workers more time off to enjoy life and do things for themselves, thereby decreasing stress and improving worker performance for the hours that they are at work. These arguments are not without their merit. However, here in the US, the people in the highest positions of power, i.e. those who stand to lose the most by such a move, would never allow the change to happen. Maybe it could happen in some European countries, but never in the US.