In fact when I was last employed (I'm now freelance, or at least trying to be freelance) I very much did not work on my OSS project, or use it in client work, because while I had verbal assurances that the company would not assert their rights, the contract I signed included an IP clause that was a little too broad for my liking.

As to the OP, It would be nice if I could figure out a way to make my OSS project earn enough to cover my rent. I would be really happy if I could get it to pay its own rent - hosting the project's home page is not $free.

(Obligatory spam of the project's home URL: https://scrawl-v8.rikweb.org.uk/ )

This isn't my experience--I've worked with hundreds of developers who do open source, and only a handful were paid in any way. I don't exactly look deeply into the finances of every open source project, but my own open source projects, I've paid to run. Now, I'm aware that my experience is anecdotal, so I'm not going to confidently say that you're wrong, but I'll say that the best evidence I have (which isn't very good) doesn't match your claim. If you want to persuade me, you'll have to provide better evidence than what I already have.

That doesn't say anything about the number of people who write OSS.

OSS projects that are created by companies as a strategic move are something completely different.