I mean not just with startups. Everyone would be expected to retire by 40.
2) Lawyers and doctors frequently work 80-100 hours a week. The amount of productive output is not 2x as much as the 40 hour week; due to fatigue and stress, the productive output only averages out to about 150% more work...in other words, a 25% loss in productivity as compared to just working 2 40 hour weeks.
3) The stress of working 80-100 hour work weeks kills the body good. Literally. It easily ages the body 2-3 years for every year on that sort of schedule. That's why all our presidents have gray hair when they retire.
4) The pyramid scheme: younger generations would have to work even longer to pay for the retirement costs of the retired. People live an average of 25-35 years (depending on sex and race) after 40. So you've made the already unsustainable equation (which sets the retirement date for federal benefits so that only 1/2 of all adults are expected to live past the eligibility date) even worse.
5) France. Enough said.
No, because someone working 40 hours a week has a large set of problems they can't solve -- e.g. anything that takes 160 man-hours to do and needs to be finished in two weeks. Someone who works insanely hard is going to be first to finish a lot of stuff, and the prize for first place can be enough to justify the cost.