I think that people making the argument of 40 hours a week is not that is it impossible or not beneficial to work 60+ hours a week at a particular time. But that it's not sustainable. It's not the same living on ramen for two years than defending that living on ramen is actually great. Again, assuming that an exceptional situation (young startup founder with no commitments) is the norm is a mistake.

Also, it is easy to think that working those extra 20 hours are a big deal, hey 50% por output! But the fact is that that's not necessarily true. Ideas need time to develop, and spending time in different activities is good for your mind. I had a shop for a couple of years, and I worked A LOT during that time (I was the only one taking care of it), and I know how founding a company sucks up absolutely all your energy and mind. But being obsessed with it is not sane on the long term. I also work right now a lot less hours and more relaxed than when I was younger and I am waaaaay more productive.