I guess that the most common case is not an interactive work, but some kind of automation systems built a long time ago, that just works, eg. Amiga 2000 running school heating system [1] or C64 running driveshaft balancing calculations [2].

[1] https://www.woodtv.com/news/grand-rapids/1980s-computer-cont...

[2] http://wbj.pl/auto-garage-in-poland-uses-commodore-64-to-run...

There is a remarkably active Amiga fan scene that continues to use these OSes for "real" and develop new applications and games. Frankly, it's amazing; random examples: https://amitopia.com/muimapparium-0-7-for-amigaos/ , https://amitopia.com/inviyya-shooter-for-amiga-500-in-develo... .

I've fantasized about doing it myself on more than one occasion. The price of new Amiga hardware sort of puts me off, but it's not outside the range I could afford. Unfortunately, I don't quite have the time right now (and, for once in my life, it's not a silly "i don't have time" excuse), so I'm watching it from afar.

Maybe some of those people who make money in retro computing? It seems to be a growing niche, with people making new hardware and software for old machines, and other people splashing their work all over the internet.

I don't know if anyone can actually make a "living" doing that, but it would be cool if they did.