I don't see why. That would mean the death of a billions-of-dollars industry, and even get rid of thousands upon thousands of jobs.
Do you mean open-source once you buy it?
Really it's simple economics: open source is an economic anomaly, the total opposite of what is "supposed" to happen, and so the industry as we know it will be wiped out... eventually. By now it's hard to call it a fad. For SOME reason programmers, and only programmers have decided that doing work for free is ok. Perhaps because only labor is involved, computer programs have no physical components or material being(notice no open source hardware

).
Nothing stops you from making money on open source software either, you can ask for donations, and you will get some, if not a lot. Not much different than a traditional model nowadays with piracy as rampant as it is. Overall better than shareware too, you don't need to withold features or weigh the program down with notifications telling you you're a filthy freeloader. That, and I don't need to remind anyone that Red Hat(and now Sun too) make money on their software through service & support, rather than selling it directly.
Then if you look back, every challenge that OSS has supposedly faced death against it has beaten, even if people have been overzealously(and wrongly) saying "YEAR OF THE LINUX DESKTOP" forever. Drivers are good for all but the most exotic equipment, they have a variety of nice GUIs, GUI-based system configuration utilities are finally getting somewhere in distros which include them, pretty much every core Application need is well filled outside of heavy multimedia editing and games, and there's work toward the former. It's pretty dumb to think OSS will have a great video editor by tommorow, but it's equally dumb to think that they will NEVER EVER GET ONE EVER LAWL.
That said....I really don't care if it takes over. It'd be nice since I'd be more likely to use and interact with open source OSes when I get into the workplace, but I won't be heartbroken if silly OSes like Windows remain on top. I'm fine with the terminal, and I'll be d***ed if I ever use a computing platform that doesn't have one as robust as Linux/Solaris again: I still can't help but think GUIs are a bit misguided, aside from the obvious need to display pictures/video... and didn't normal people get along with DOS when it was all they used in businesses?
