> A business making revenue of $20k a month must be a very small idea, for example a service that pings some servers and alerts customers of downtime or maybe some kind of file storage.
I don't think this paints a correct picture. For example, there are many developers out there who'd love to work on FOSS 24/7 while having some income from a simple side-project like you mention. But many of these developers still struggle, which says that it just isn't that simple.
You cannot be working on FOSS 24/7 and also be working on a side project to make money. If it were that easy, many people would do the simple side project and then skip the FOSS part altogether and go do something fun.
FOSS is what you can do when you're no longer tied up in the mundane day to day tasks of running a business; when you have employees who keep the business processes you put in place running while you simply make high level decisions and provide capital.
And of course these small kind of businesses rarely last forever. Instead of thinking long term, think of this small business as a temporary opportunity to extra value from a market while it is still there. Eventually the opportunity is exhausted the way a mineral field gets harvested and you must find a new opportunity to mine. There's not much time to do both things well.
My advice to developers who want to build FOSS for a living is to make money from the software itself by providing implementation and consulting services around it. This way your interests align well with the things that pay your bills, and you can excel at both simultaneously.
I don't think this paints a correct picture. For example, there are many developers out there who'd love to work on FOSS 24/7 while having some income from a simple side-project like you mention. But many of these developers still struggle, which says that it just isn't that simple.