Earlier quoted context omitted.
The humblebundle sales showed clearly that linux users are ready to pay much more for video games than windows users are. I have personally used hundreds of dollars on linux compatible games.
When was the last time you paid for an indie humble bundle though? I think it proved that for a short moment a very small percentage of users were supporting games at a higher rate than the majority in order to support a cause, but once the novelty wore off they went back to not caring. I just don't think there are enough Linux users out there that are also regular gamers to make Linux support worth it for most title…
Some years ago, but that is explained by a sharp decline in the quality of games included in their offers (which was about the time they dropped ensuring that their bundles were compatible with the three platforms). A better question would be when I last spend money on (game) software for linux. The answer is 28 hours ago.
I concur that the market is small. But when your sales are small too, it is not smart to throw away access to a market with a higher proportion of dedicated users and consumers. From the perspective of tech, less time may have been "wasted" on infinitroid if the developer used more off the shelve solutions that would help with multiplatform targeting.
In the end we are just basing our arguments on our gut feelings. I doubt very much that Valve would push as hard for gaming on linux as they are doing if they didn't see the potentials of sales for the platform in their data. But time will tell.