I get your point, but it might be you do not want to build a $1B company.
Y-Combinator is designed to build $1B+ companies, however, it is not designed to not build $1B companies at all. So if you don't want to do that, yes do not work 80h/week. However, this way, you are also very unlikely to build a company of that size.
If you want to build a huge tech-company like Uber, Twitter, AirBnB, you will need to spend all your brain capacity for what you are doing in order to make a company like that happen, you will simply need to sacrifice a lot, friends, sports, having a girlfriend, sometimes even sanity. Most people would never do that and that is fine, however, there are some people can't imagine doing something else, because they really, really want to change the world and there is nothing else that would give them more satisfaction. This is not possible as a side-project and the evidence is that the huge companies we have these days were not built part-time. They were built full-time, probably double full-time.
Hope the other point of view makes a bit more sense to you now.
The parent understands what you wrote just fine. What I think you misunderstand is the parent's point. The "$1B" has to come from somewhere. The parent's point is that in the Valley/VC bubble, that $1B isn't primarily from economic activity in the traditional sense that a business provides a good or service to enterprise or retail consumers in exchange for money, but is instead primarily from additional rounds of fundraising from other investors in later series.
So I'd modify your first sentence to make it more accurate: "It might be you do not want to build a company with a $1B value that is largely paper-money based on investor provided funding."
Y-Combinator is designed to build $1B+ companies, however, it is not designed to not build $1B companies at all. So if you don't want to do that, yes do not work 80h/week. However, this way, you are also very unlikely to build a company of that size.
If you want to build a huge tech-company like Uber, Twitter, AirBnB, you will need to spend all your brain capacity for what you are doing in order to make a company like that happen, you will simply need to sacrifice a lot, friends, sports, having a girlfriend, sometimes even sanity. Most people would never do that and that is fine, however, there are some people can't imagine doing something else, because they really, really want to change the world and there is nothing else that would give them more satisfaction. This is not possible as a side-project and the evidence is that the huge companies we have these days were not built part-time. They were built full-time, probably double full-time.
Hope the other point of view makes a bit more sense to you now.