When I open sourced POP Forums around two decades ago, I honestly did it in part because it wasn't something that I could sell anymore. The window of opportunity for selling niche software like a forum application was pretty short. I originally built the thing because I wasn't satisfied with existing options, and I wanted to have the ability to integrate with other things. Since I was maintaining it anyway, I may as well have given it to others to use. The fun thing about it is that it has since been translated into five more languages, and the packages have been downloaded thousands of times.
These days, I have the added motivation to keep my skills sharp and maintain some amount of street credibility, since I mostly manage people and process and don't write code in my day job. But more and more, I'm taking advantage of existing open source software in ways that lead to cost saving, shortcuts and really robust solutions that wouldn't be possible otherwise. Recently I've been messing around with Keycloak for identity management, and KrakenD for API gateway orchestration. These are fairly mature products, yeah, I think they qualify for that term, because they're so robust and well thought out. These big projects are often backed by companies that offer paid support or consulting for them, so there is some incentive to maintain them.
I feel like, however small the user base might be, it's important that I'm giving back to that ecosystem. That's why I imagine that I'll continue with my stuff for as long as I can.