Each story you put out into the world, whether it's through a publishing company or indie published or even free on your website/blog, is like buying stock in a company.
Some stories will do real well, while others may only sell a few copies a year. Some, unfortunately, may not sell at all (not including the ones that are free, of course, which can pay off indirectly).
Very few titles will be able to carry the weight of your financial success all by itself. Yes, yes, we've all heard of Andy Weir's The Martian and J. K. Rowling's Harry Potter series. Those are anomalies. The majority of writers will have to write a plethora of books in order to pay off for years to come--including after you are deceased for the duration of the copyright.
If you treat your publishing empire like an investment portfolio, each story can pay off small individually. Together, you can earn more and more with each subsequent title. And taking each title into different mediums, whether it's print, digital, audio, film, foreign, each one can be split into even more revenue streams.
This makes me smile when I think of how these stories will pay off in the future--even in the decades to come when my children are out of the house.