That seems about right considering all other OS usage metrics around.
Even if you say "oh but most people with linux dual boot windows" and so on, maaaybe you're underestimating the value of actual linux installs by what? 20%? Lets go crazy, lets say that there are actually twice as many Linux users as the survey says (ie, the metrics are terribly wrong and all Valve employees should get a year long statistics lecture). Even then its just 2.3% of the total.
I'm guessing that if SteamOS picks up steam (pun intended) and everything goes right, you might see 5% of Linux usage... Someday... Maybe.
It's still a lot though. Say a modern AAA game sells really well, 10 million copies. Windows only game (Skyrim sold like, 8 million units the first month). Now say you could increment your potential sales by 5% by releasing it for Linux (actually its a bit more than that because its (95% of a total minus OSX usage) vs (5% of a total), but bear with me).
That's 500000 potential copies sold (and to be fair, if each pirated game is considered a lost sale, I can consider each Linux user as gained sale). Now, I won't matter much to an industry which is used to double the sales of each of their games every year, but that won't keep up forever, and that 5% someday will actually be an attractive number.
That's how I think it will be IF everything remains more or less the same, ie, not taking in account some technological revolution in between (like smartphones).
