I hate to burst the bubble here, but everyone I know at Microsoft Japan is still working at least a 5-day week.

It's just that they don't come into the office on the 5th day.

This is actually probably the key result from this experiment. Working at home that 5th day, allows workers, especially in Japan, where people feel a huge obligation to just hold down a chair even if there isn't work to be done, to work flexibly in a manner that works for them.