Catastrophe, as if Valve would suddenly go out of business?
Even if they would, they would have to go out of business in an orderly fashion, meaning they'd have to provide a tool/patch/client update that allows you to play all the games you already got downloaded. Plus at least a 1 month warning so you can actually download all the stuff you need before the servers get shutdown.
The more likely outcome though would be that some Chinese IT giant just takes over the remaining assets.
The most likely outcome though would be that Gabe ate the Internets.