So what is the solution? Is the author demanding that people work for them for free to do the sustainability for them? Because that sure sounds like the only way to "resolve" the complaint.

"start treating it as what it often is: a refusal to do the harder social work in #FOSS"

Your ending is missing something... "a refusal to do the harder social work that I want you to do in #FOSS".

But I didn't promise that. Nobody promised that. FOSS is an unparalleled gift of free work and not a single line of it has formed an obligation on my part to help anyone who wants to come along and make it do something different. You are welcome to do that, but I have no obligation on any level to come along and help you "sustain" your own work. No legal obligation, no moral obligation, no community obligation no reciprocal obligation, no Kantian imperative obligation, no obligation whatsoever. If anything, you owe them, not the other way around; any other read of the ethical situation is utterly absurd.

You want "more social work" done, you feel free to do it. Don't be shocked when I'm not interested in helping.

This is just a demand for more free work from people who have already handed you the result of more free work than any other collection of work in human history. It is deeply ungrateful to demand yet more.