The mistakes of the Linux porting companies isn't that they ported old games. It was that they charged brand-new prices for old games. If I didn't already have it and if I cared to return to Linux as a non-server OS, I'd happily pay $20 for L4D2 or HL2 on Linux.
The purpose of Valve in this endeavor is also not exactly "make more monies by selling to the vast 1% of the market that is Linux." The project is partly just Gabe being very very interested in Linux and, being rich and having an entire company of his own that makes games, he can damn well port his stuff to Linux if he wants to. And he does. So he is.
Just like most of you keep using Linux despite its statistical unpopularity and yet still consider the OS fantastic, it's quite possible that Valve will consider Linux ports a success simply because they exist, even if sales are low. Ideology and profits rarely have anything to do with each other.
Also in terms of repurchasing power, keep in mind that some companies with a fanatical following do very successfully manage to peddle the same crap over and over again. There are big titles that make a splash and disappear a few months later (remember Diablo 3? me neither) and then there are titles that the gaming community just never stops being in love with (why hello there, Starcraft).
Don't get me wrong; new titles are where the money is at in general, and if you care about sales you need to be releasing new content. It's not at all necessary to push a new a platform or technology, though. With the right product, you can keep on peddling the same fossil over and over again and be successful.
p.s. Yes, it is fucking retarded that you have to pay for DLC on the XBox that is free on Steam, which is entirely due to the ridiculous fees Microsoft charges developers for updates and DLC. It's all Microsoft's Live team being a bunch of douchebags. Don't even get me started on Games for Windows Live. That entire department needs to combust, now, without any warning to the people who work there.