You're assuming that work is fungible, and that success is a monotonic function of amount of work.
There are certain types of work that I don't have the mental energy to do when I get home from a full workday, or at least not to do well. And, honestly, if I try too hard, I risk lowering the quality of my work the next day and burning out completely at some point. And burnout is a fairly large reason that startups fail: "But I think the underlying cause is usually that they've become demoralized" (http://www.paulgraham.com/die.html).
There are other types of work (e.g., practicing piano or voice, two pursuits where the amount of time spent is pretty steadily and monotonically correlated with success) that I can do just fine. It seems more than likely that, if we get it to the point where you can do a successful startup on 5 or 6 hours a day, spending an additional 5 or 6 hours per day on the startup is unlikely to make you more successful, but those same 5 or 6 hours can be productively directed towards music practice, raising children, or whatever else you want to do.
(There's also a line of argument about parenthood being of intrinsic importance, above success at startups, success at music, etc., but I'll let someone else argue that point.)
There are certain types of work that I don't have the mental energy to do when I get home from a full workday, or at least not to do well. And, honestly, if I try too hard, I risk lowering the quality of my work the next day and burning out completely at some point. And burnout is a fairly large reason that startups fail: "But I think the underlying cause is usually that they've become demoralized" (http://www.paulgraham.com/die.html).
There are other types of work (e.g., practicing piano or voice, two pursuits where the amount of time spent is pretty steadily and monotonically correlated with success) that I can do just fine. It seems more than likely that, if we get it to the point where you can do a successful startup on 5 or 6 hours a day, spending an additional 5 or 6 hours per day on the startup is unlikely to make you more successful, but those same 5 or 6 hours can be productively directed towards music practice, raising children, or whatever else you want to do.
(There's also a line of argument about parenthood being of intrinsic importance, above success at startups, success at music, etc., but I'll let someone else argue that point.)