The monster hits in PC gaming can sell over a million copies, but these are pretty rare. The Diablo games, The Sims, Myst, Rollercoaster Tycoon, Half-Life, the Quake and Unreal series have all sold over a million copies, but these are the exceptions. Most PC games actually sell pretty poorly. If you can move over 100,000 copies, you've got a hit. 250,000 would be a very solid number.
So I guess the answer is that while companies like Valve would probably like to sell a lot more, they could tailor their games to that 250k and still turn a decent profit (you also have to extrapolate -- how many people bought HL but didn't answer the survey and have similar specs?)