Open source enables a majority of web servers, a majority of email deliveries, and many other businesses, organizations, and personal pursuits. Any improvement in technology that permits business to function more efficiently means the economy runs more efficiently. In this case, open source enables business to spend less on software and to have better quality and more control over its software. The money that is saved on software does not disappear; the people who save it spend it on things that are more important to them.
It may be that open source eventually causes a reduction in demand for proprietary software. This would not, however, reduce the demand for programmers, because the demand for software in general would not decrease. The displaced proprietary programmers would move to an organization that can produce open source software in an economically successful manner.