It seems to me that the common (Western) standard of ~ 40 hours a week is something we inherited a generation or two ago, and consequently is considered to be normal, average, ideal, expected, realistic, fair, reasonable, etc.

As noted, societies have previously had very different norms which have been changed without societal collapse.

Applying the scientific method to the ~40-hour assumption is something we should, as rational actors, embrace. What few experiments conducted so far suggest at least some positive outcomes, but more importantly indicate more experiments are worthwhile.

Personally, submitting to 5 x 8 hours a week, 11 months a year, repeat until I'm too old to continue (or indeed to anything else) just because that's what dad did, isn't a compelling case.