When I said "this is unrelated" I was commenting SOLELY on your writing style.
> How does one afford to work for free?
This is exactly my point. The work isn't done for free, the person is spending their own money and time which takes away from a limited pool of resources they own. If they're insanely rich, they could probably "afford" to do this work until they die.
But you're making a mistake in your reasoning relating "volunteers", "free work", and "day jobs." Here is what I think you are missing in this assessment: A worker for a company/day job works with an obligation via contract for compensation for their time from their employer. A volunteer works without a contractual obligation of compensation for their time from the community that benefits from their work. In this latter case while there is no contractual obligation for a society/community to compensate the volunteer, it does not forbid it. Does someone who works as a volunteer search-and-rescue deserve to be compensated? I'd say yes, in fact, they do. They are providing a service.
Now I'll get ahead of the next possible argument. "But there's not enough work or compensation for them to make a living!" This is two parts:
1. For not enough work - This is only true because of the example chosen and our human tendency to draw broad analogies. There can definitely be enough work in multiple domains (and especially in software) but also what about volunteer firefighters?
2. Not enough compensation - This is because people with the means to compensate the work, simply are not doing that. And it's not a good faith argument to tell me that in the original case enough people with enough money aren't using the project to compensate it's continued development and maintenance.
To sum up all of the above: Yes, work like this is volunteer work, and it says a lot that societies and communities do not compensate this work. Simply because they don't compensate that work doesn't mean it's not able to be compensated. And there are key differences between this relationship of work and compensation that make it different from the colloquial "work" as in a day job. As this is an entity reserving your time under contract.
Now for burnout as a double-worked programmer. I think you're right on these two points. Obviously the second situation is not ideal. If someone wants to do it, let them. There are plenty of open source projects still maintained by stretched thin developers. Is this a tenable solution long term? No in the vast majority of cases and that's my point!