While most have the goal of replacing their job income with some sort of capital income, I've found that on top of that income, during the ~3 years between the physics and the quant job I always had 1-3 projects that also made money; enough to live on actually.
I've posited that if one is serious about developing multiple skills, making extra money is almost inevitable. For some it may even be a primary strategy (see "How to live without a salary"); essentially the working man strategy of chapter 3 in the book.
However, am I correct in this presumption? Will everybody eventually find opportunities to make a quick buck from their hobbies (e.g. the aerobics fanatic eventually being asked to sub for some classes) even without trying very hard?
Or are there retirement occupations (TV watching, marathon running, ...?) where this is NOT a given?