The test of time and writing a book to make you proud.
To be a professional writer, you have to be able to live from what you sell. This is a general definition.
The segmentation criteria are irrelevant. Genre doesn’t matter. The literary qualities of your work don’t matter. You might criticize a vampire novel because it is badly written although it sold like crazy and there is a movie based on it. You could be right in your critique, but truth is the author is a professional writer. He/she sold books to many people and you can’t deny this. He/she is a writer with a well-defined target audience.
Nobody says you couldn’t do it as well. They will criticize you or not. People will love your book or not.
What I’m interested in is if you are proud of what you published? Is it something that will stand the test of time? Is it something that your kids or nephews could read when they will be at the age the book was written for? Your husband, wife, or significant other will recommend your book to colleagues at work?
Do you feel satisfaction for what your book has achieved in teaching readers something good, changing the world?
Is it a gratifying experience to hold a hard copy of your book in your hands? Do you feel proud of your creation and say: “I wrote this book!”?