Please define "unsustainable". It has worked great for decades, both for the free market side, and for the FOSS community.
It has certainly 'worked great' for leeches, if you ignore bombs like this logging bug destroying Western civilization. Can you explain a bit more how it worked great for the bulk of maintainers / authors who don't see any return on their work, burn out and have to do something else?
Yes, it would be very good if more people started to contribute to software they depend on, but to call them "leeches" is not only against the spirit of free software, it is counterproductive as it will probably lead people to the idea that proprietary/closed source is better.