Yes, we could call work '16 hour weeks' and maintain a 1940's standard of living. But to make progress, we need 40 hour weeks.
At least for the time being, we've come to resonate around 40 hours as being 'how long someone should work' - and so we will work those kinds of hours.
Sure - we could all work less hours, but it would mean another kind of social contract.
You can see this in Europe vs. USA vs. Asian attitudes.
And yes, 'more hours' generally does mean less productivity per hour, but you still get more output.