The market value of a program is its value as a saleable commodity. The monopoly value is the value you gain not just from having the use of a program but from having it be unavailable to your competitors. This starts with the market value and monopoly value of software going to zero because of all the free sources out there. Use value alone doesn't attract enough consumers to support software development. The commercial software industry collapses.
Programmers starve or leave the field.
Code written for resale is only the tip of the programming iceberg. Most vertical code is integrated with its environment in ways that make reusing or copying it very difficult. Thus, as the environment changes, there is a lot of work continually needed to keep the software in step. And it will still need to be done, even when most software is open-source. As we'll see, the actual exposure is probably much lower, and there may be none at all.