I am afraid there's nothing people can do about this issue: according to https://github.com/osxfuse/osxfuse/blob/master/LICENSE.txt the project is BSD licensed and AFAIK closing the sources of a BSD-licensed project ("re-licensing") is allowed.
Yep, he's allowed to do this and people basically have to suck it up.
This couldn't have happened if the code was GPL-licensed.
This might be a good occasion take a moment to think about all the BSD-licensed software you're currently using, and imagine it disappearing just like this piece of software. Then go back to your repository and re-license as much as you can as GPLv3.
A key problem is getting people to pony up. People are all for "the community" and the maintainers doing right by "the community" when that means they (as a member of the community) get the benefit of the code. They are often less vocal about being part of the community then the project maintainers ask for financial assistance, generally assuming that other people will chip in so it'll be OK if they don't.
This isn't just commercial interests: this is private individuals too. Welcome to the idealism of F/OSS rubbing up against less altruistic parts of human nature.
While I'm judging others I'll have to hold my hands up and say that, while I have helped out some projects in the past via both bug-fix contributions and small financial ones, and having opened some of my projects in the dim & distant past, I don't think I can claim to be a net contributor by any wider measure.
I don't see this as idealism vs a selfish human nature, not directly at least. The maintainer has been delivering a low level open-source product within a proprietary operating system. That's got to be tough, and as the article states, it's the main source of hurdles to others taking up the project.
There is one huge thing that can be done about this. Push the commercial users of osxfuse to penny up and share some profit. According to the article they are using osxfuse for free and not contributing any code back either.
If you use software that is dependent on osxfuse....send an email to your sales contact and let them know that they need to contribute back to the community.
If he wanted to switch to GPL, he wouldn't have to relicence all the code; just the code that is by him alone. The contributions by others are in a permissive license that is compatible with the GPL, so they could remain in their current state; but the whole project would contain GPL code so anyone redistributing the project would have to do abide with the terms of the GPL.
Yep, he's allowed to do this and people basically have to suck it up.
This couldn't have happened if the code was GPL-licensed.
This might be a good occasion take a moment to think about all the BSD-licensed software you're currently using, and imagine it disappearing just like this piece of software. Then go back to your repository and re-license as much as you can as GPLv3.