The thing about Carmack and Linux is, that everything he says is based mostly on the QuakeLive port id did a while back (edit: he says this here http://youtu.be/wt-iVFxgFWk?t=45m26s). The port was released as an afterthought and quite a while after the Windows version), it wasn't publicized anywhere so naturally, people ignored it. From this experience he concluded that Linux isn't financially viable and/or there are no people interested in gaming on the platform.
Now I'm not saying that Linux is financially viable, I don't know that. I just think that
- a lot of people do pay for games on Linux (look at Humble Indie Bundles, look at the indie titles or the recent titles on Steam)
- I know people who pay for games but are stuck on Windows because they play games. My guess is that these people would ditch Windows in a heartbeat if their games were playable under Linux.