You can sell support. You can sell consulting. You can do it the curl way and make yourself hired for developing your useful thing full time.
Not saying it's easy. Building a product and selling it in a way or another is not easy.
As a sibling comment say, at least the source is here, if you've come to rely on this unmaintained piece of code at least something is possible.
But I was mostly reacting to the association of "Closed Source" and "For sustainable business", like if sustainable business was not possible with open source. It is. Many companies succeed in this.
But it can. I mentioned Igalia and MediaInfo, and developed on this at https://news.ycombinator.com/item?id=38556450#38557836
I don't really know about games and good developer tooling, so I can't speak for these domains. Games are probably particularly tough. But I really think people may pay you for implementing features they need, buy support and for doing custom dev work. That last one takes your time out of your main product though, but if you manage to do custom work on top of your main product, it can still be a good deal.
With commercial desktop software at least there is the shadow of the law, thus some companies do survive despite piracy.
Hence why digital stores are so beloved by companies selling desktop like applications.
I bet there is hardly any company sustaining their business from sales on F-Droid.
At least with OSS, you have the power to patch it and recompile yourself.
I have also seen models where the source is made available just before that happens. For eg: https://www.placemark.io/post/placemark-is-winding-down
I've seen so many OSS projects abandoned because the maintainers couldn't find the time, given they had to work full-time on something else that would allow them to pay the bills.