Long ago, when bricks and mortar publishers ruled the publishing world, those publishers decided who got published and who didn’t. In doing so, they decided who made a living as a novelist and who didn’t.
Now that anyone can self-publish, a lot of authors have decided that the path to fortune requires giving away current books in order to make money on future books. That means that there are so many free books out there that readers can read books without ever having to pay for them.
That’s had a negative impact on what all authors make. The average income from writing novels and novellas is now between $2000 and $5000 a year. That’s not a living wage.
However, the light at the end of the tunnel is marketing. If an author is willing to learn how to market— and, once they’ve learned how, is willing to market hard— then he/she can make a living wage writing novels and novellas.
Most authors I know say they spend more time marketing than they do writing. I believe it.
So, in conclusion, it all depends on how much work you are willing to put into getting your novel and novellas noticed by people who are willing to buy them. On how hard you are willing to market.