Windows 8 isn't an existential threat to Valve because there are some UI tweaks, it's a threat because Valve is tethered to Microsoft, and for some, Windows 8 has not inspired much confidence.
Thankfully, we don't have to guess about this particular metric, or use silly results based on web-surfing of certain websites. Almost 9% of Steam users are already on Windows 8:
He's not worried about everyone buying Windows 8, he's worried about no one buying it:
http://www.xbitlabs.com/news/other/display/20130204235238_Gl...
Windows 8 isn't an existential threat to Valve because there are some UI tweaks, it's a threat because Valve is tethered to Microsoft, and for some, Windows 8 has not inspired much confidence.