More like a 3rd of that, unless you assume that every man, woman and child regardless of age, owns and operates a PC.
Desktop Linux a marginal market niche of enthusiasts who will continue to consume it regardless of competitive advantage (i.e. either because of politics or because they need a commodity Unix-like OS on their desktops). That means that as a commercial alternative Linux is dead, unless your target market is that same enthusiast market --after all people still sell software for the Amiga.
The larger question that I don't see debated anywhere is: is for-profit open-source software development dead? Exclude the service providers (like RedHat) and 1-2 politically motivated success stories (Firefox; more of an ad service than a software vendor anyway), and who are the software vendors who are making money off of open source?