For manual labor this would make a big difference. For intellectual work, I suspect it would probably not lead to much less work being done. Just because people are at the office 8 hours a day, doesn’t mean they’re getting 8 hours of work done. I would be curious if people would get as much done and waste less time sitting there or if the sitting there time is the same, percentage wise.
My current job I only work 35 hours a week and it's great. They pay us for an hour lunch basically and I just take it at the end of my day, thus shortening my day to 7 hours.