I suppose there's a theoretical limit at which the marginal utility of an extra dollar is less than the marginal utility of the time I give up to earn it. But I certainly haven't reach it yet. If your friends have, I want to know where they work so I can apply.

This depends on one's utility function... and all the cognitive mistakes they make evaluating it.

For someone who feels their free time is very important (even if they later waste it procrastinating online) and who has covered their basic expenses and can't imagine an attractive enough use of a marginally more money, it is rather easy to get to this point.

And by the way, the ability to fund your goals, such as writing and publishing a book, is pointless if you don't have time to actually write the book. I assume your strategy is to make money now and write the book later. This is a valid strategy, just don't forget to stop making money and start writing the book, when the right moment comes.