It seems to me the author's point is: Why do people who reach a certain level of wealth keep grinding away at jobs they may not like? For the sake of argument, lets exclude those who shift and work on what they care about. I think he's talking about those who do not necessarily love what they do. This group would be millionaires, and using his numbers are the top 0.005% globally (40M/7B). His answer is that they continue to strive for money because they look towards the next class above them and to provide for their children.
I agree with the author in that people are seeking higher and higher status, and I agree with a peer comment that people get used to luxury. But I would add that that this striving for climbing the class ladder is both endemic in the culture of the US as well as accessible. In many other places in the world there are stricter controls on upward class mobility. I'd posit that people continue on in this because they don't really create a philosophy of life or pay that much attention to how to live until they are much older. It's easy to just keep doing the same thing, especially if it provides a luxurious life. It is a subjective value statement on what to do when you reach that point that is clearly different for everyone.