Because ... only certain types of people have the ability to program. If true (and I believe it to be true) then programmers will forever command higher than usual salaries. And the more demand for programmers, the better-off will be programmers. And the more use of software, the more demand for programmers. And the less expensive is software, the more wide will be the use of it.
I believe that there is sufficient evidence to say that Open Source and free software lowers the cost of production of software, and hence will ineluctibly raise the salary of programmers, even as these programmers give away more and more of their software.