“Giving away your work for one and all to come and take is, ultimately, unsustainable.“

I don’t think this statement is accurate as there are decades old projects (linux/gnu, apache) to dispute your claim. I think this is possible because software actually is possible where a few individuals can have great reach with part time or employer funded time. And also because companies can benefit by funding and releasing activities that aren’t core to their business as either advertising or inclination by developers to be altruistic or other reasons (MSCode, Google stuff, etc).