They misunderstand something fundamentally: Free Software is not about making money, it is about freedom.
Making money with freedom did never work.
But making this world a better place works with freedom.
This means free software cannot every be sustainable (other than if it had corporate backing of some kind). Making money (or more correctly, rewarding those who create value) needs to happen, otherwise, the machinery that lead to the creation of the software will eventually wear away. After all, bills needs paying.
Then Art is not sustainable either, because very few artists earn a lot of money with their works. The masterpieces of Art were seldom a financial success.
The oldest known art is about 37000 years old.
That is much older than money and a proof of great sustainability.
Non-profits are not about making money but the underlying sustainability of their cayuses still requires a sound financial framework (no mission without margin). This is not really any different form the free software movement.