It means if you want more labor than can be accommodated within a 2 day work week, you should hire more people.

Sure, I agree with that. However, we also need to consider what a "net decrease in productivity" actually means for the population as a whole, and whether it's something we want to accept as a trade-off for more free time. Briefly, we can collectively choose to work four, three, or even two days a week, despite seeing a decrease in overall productivity. However, a decrease in productivity means that stuff like clothes, transport, food, IT services, and pretty much everything you can think of that someone has to produce becomes more scarce.

You basically need to answer the question of "would you prefer two days off per week with current access to goods and services, or have more days off with reduced access to goods and services". Of course, there may come along technological innovations that change this in some ways, and there are studies showing that a lot of people can be sufficiently productive on a four-day work week. On a society level, I still think the point stands as an overall tradeoff we need to consider when talking about whether we should reduce the work-week.

My point is that it's not just a "capitalists are bad, and we're owed more free time" thing. If we produce less, then goods and services become scarcer for everyone. I would say the distribution of wealth in society, and how it's shifted the past 20-50 years is more concerning than the fact that we're working the same hours as we were 20-50 years ago.

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.

I would argue that having more hired staff on rotation would be more productive, even with fewer workdays by any one person. By having more people who have experience with a given task, should any one of them become unable to work, someone else can pick up the slack until things have returned to normal.

When a company relies on linchpin personnel, the loss of a person could be devastating, since the company will have to find a capable replacement to hire. That means interviews, introductory payment packages, hoping that delays don't cause further damage, establishing the new hire in the workplace, and so on.