Since manufacturing isn't as big a part of the economy as it was, [...]=/=
as you say...mfg isn't important anymore [...]
As to how it would help: There are close to 20M people in the US who aren't working, or aren't working as much as they want/need. I think you'll find that there are many more, larger, boatloads of people like that than white-collar people who have to work 80 hours a week. Shifting to a 30 hour week would enable more people to be employed (to get the same [or increasing] amounts of work done).
But I guess since history tells us that the 40 hour week destroyed the US economy, well then ... :-/
Cheers,
Scott.