Isn't there any alternative possible? This seems like a huge opportunity if solved correctly.
One is that we collectively decide to stop shaming software developers for having the audacity to want some level of ownership over the product of their work. We don't shame authors for wanting copyright on their books; we don't shame musicians, artists, designers, or aerospace engineers for asking for some copyright protection for their creative babies. Yet when a software developer does it: fuck that guy! He's trying to take control of what's running on your computer (or the internet server that you're sending requests to ...). Nobody throws a hissy fit when J.K. Rowling has (gasp!) copyright over the Harry Potter books that are sitting on your Kindle. It's your Kindle! Shouldn't you have the right to copy off the words in the books and re-sell it to other people for much less money, undercutting Rowling? How dare she try to get some legal protection that says you can't do that! It's fucking ridiculous when we talk about authors that way, but somehow it's OK to talk about software developers that way.
Do you think "open source authors" would make a living from their books? It's already difficult enough for new authors to get any notice; how much worse would it be if prominent authors (who were already rich) came out and founded the "Free Books Foundation" that comes out and says every young author who's trying to sell her books for money is being a greedy asshole and we should fight against them and every author needs to spend a significant portion of their free time contributing to "open books" or they're assholes?
Of-fucking-course it's not sustainable. That's because it's always been OK to want copyright on your creative work. I'll be the first to say patents are a huge problem right now and we might be better off without any patent law, but copyright is not the same. Yes, the terms are way too long, and the family of Marvin Gaye proves that "copyright trolls" are possible, but the fundamental concept of copyright is actually critical if we want creative people to ever get a paycheck.
The other alternative is Universal Basic Income, so that making "below the minimum wage" doesn't mean "fuck you, you get to die sick and homeless in a tent on the side of the highway". Then people could actually just contribute to OSS because they want to.
I also think you're misunderstanding the FSF. Nowhere do they say that selling software is bad, in fact they explicitly say that you can sell your open source software, and also note that it's free as in freedom, not beer.
The real argument is that open source is not a business model. Just because someone writes OSS does not mean they will automatically make money, because again, it's a distribution model, not a business model. There are plenty of OSS that make money because they focus on the business and marketing side instead of only the developer side. That most devs don't want to do that and prefer to write code does not justify their crying when no one pays for their free product.
And I think that's the core of the issue. If software, after being produced, can be distributed to everyone at only the cost of duplicating it which is pretty minimal then it is clear that putting these monopolistic restrictions is hindering progress. And Harry Potter is actually a very good example, because J.K. Rowling has always been very against any form of derivative work such as fanfiction and as copyright laws extend then she is given more and more power to control this product that is essential nothing but information. So, I really think you're missing the mark here because copyright laws are precisely something people that are for OSS decry.
And from there is where this critique arises. How is it that in this system where we have the capabilities of distributing information to anyone that wants it at almost no cost the only way to promote the discovery and creation of information is by creating artificial monopolies. When you say it's OK to want to copyright creative work, I really question that, is it really okay? Is it really desirable? Is it good for society? If our the only solution our socio-economic system gives us is this, then maybe we ought to question those fundamentals as well. Information is not going away and it only gets easier and cheaper to reproduce; so if it's pushing the system in this way then maybe we need to change the system.
I might go to fosdem this winter. If I do, it'll be the first such event in almost 20 years: https://rant.gulbrandsen.priv.no/openssl-hate