If I write a book and release it for free, I may get a lot of readers willing to read it. The moment I say, “sorry readers - to read this you must pay me!” suddenly I am now competing with all forms of paid entertainment.
A lot of the draw to open source is that it’s free. A lot of the allure to open source devs is knowing that tons of people are using their software, the license means they can’t be sued, and they don’t have any of the headache of running a business.
Once you believe “I deserve to be paid for this” I have a lot of counterpoints.
What are the free alternatives? What am I getting for my money? What support do you offer?
In a world where people agonize over Netflix going from $9 to $14, you need to really provide a lot of value to get someone to open their wallet.
No one deserves to be paid just because they did something popular. You deserve to be paid if people are willing to pay you vs. all the other things they can spend money on.