when the 5 day a week, 40 hour work week began there was a specific level of productivity. As technology increased the output increased.

Exactly, so following this argument, we can choose between living at our current (increased) productivity level (40 hour weeks), or trading off the technological advancements for more spare time at the cost of going back to the productivity level we had previously.

I won't argue for which of these two is "correct", I think the tradeoff between free time vs. more access to goods and services is considered very differently by different people. However, I do think that a major problem we're facing today is that the increased productivity we've had the past 50 years due to technological advances has benefited the wealthy far too much, at the expense of everyone else.

I think it's more fruitful to first try to take care of the wealth distribution, such that we can actually see the quality of life our current productivity level can give everyone. Then we can make an informed choice regarding whether we want to reduce the productivity in exchange for more free time.

I would assume that there's a balance to this. At some point the reduction of hours will result in a loss in productivity. You can do 5 days of work in 4 days if you're better rested and more focused, but this might be less true in 3 days. I mean if studies show that there's isn't a dip productivity and that it improves well being, then sure, that would be great but I think it's likely than a 4 day work week.