Your "unlikely situation" is literally-not-figuratively how most authors you've heard of--no, not the GRRMs of the world, but the folks who sell books steadily--keep the lights on.

The long tail exists and is very long--longer today in a world with Amazon--and that they don't have to write a screenplay adaptation and can instead eat and keep a roof over their heads while they write the next book is a feature, not a bug. It accrues over time and the gap between personal prosperity and precarity grows over time because the back catalog keeps selling.