When Stallman thought seriously about open source in the 1980s, it wasn't because he was the only one to ever imagine a culture of sharing around software and information. I think he was reacting to a sense that an existing culture was being erased by growing commercial interests.
One could also argue it originated with gentleman-scholars circulating letters about their new physical science discoveries during the renaissance and feeding into the subsequent industrial revolution. But, I expect it is an emergent property of humanity which is probably exhibited throughout history and pre-history.
It's essentially the same idea of commons and culture applied to yet another domain of knowledge and technology. Was it sustainable for the first farmer to teach his techniques to another? For one chef to teach another how to prepare his dishes? Open source software can be sustainable as long as there are needs and resources available to produce and execute software that does something different or better than could be done without the software (or at a lower cost than other available options). The subsequent sale of additional copies need not be a motivating factor at all.