No citation needed, it's an assertion based upon the most rudimentary understanding of economics. Of course, I might be wrong.
Sure. But there are other ways to pay programmers than by the sale of proprietary software.
You are correct. However, all of the points referenced therein make presumptions about open source software that was already mature. How do you think such open source software becomes mature? By developers with programming skills. How do those developers gain programming skills? Hint: not solely through open source projects.
And you don't think the benefit flows in the other direction too?
Of course it does. There are many mutually-beneficial relationships between open source projects and commercial entities that use them.