The reality is, the Linux market is vastly underestimated.
Market share based on sales is meaningless, and heavily biased to Windows. For one, Windows sales are counted when a vendor builds a machine. It may be in a box in a warehouse and never see an end user, but it's still a sale for Microsoft (to the vendor, their customer).
And, it doesn't account for the Windows machines that are immediately wiped and replaced with an alternate OS of any sort.
Since linux is available for free (and in many flavors), the vast majority of Linux boxes are completely unknown to anyone measuring share. For example, the server and desktop running in my house both run linux (opensuse) and were built by me. In fact, aside from laptops, every box I've owned for the last twenty years I built myself, and only one of them ever ran Windows. For the last 10-12 years, they've run linux (DOS or OS/2 before that). I doubt any of them counted in anyone's market share. Except the one Windows box. And the one copy of Win3.1 I bought, but never ran as a primary OS.
That one Windows box was because I was/am a gamer. I built it as a Win box explicitly to play games. I originally got into gaming with DOS, before Windows took over. I have never been able to stand Windows enough to put up with it when I don't have to, so when gaming went Windows, I turned to consoles for gaming. Building that box was a return to gaming, but a shortlived one. Eventually, Win's random lockups drove me to quit. Booting linux on the same box ran fine for years.
My 17 year old grandson is an avid gamer. Heavy Xbox gamer. Also a PC gamer. Several years ago, he was running linux on his machine. The only
reason he's not still, is gaming. And, with Steam coming to linux, he may move back.
a market for games on linux, its just not visible as such. That market has either given up gaming, capitulated and run windows, or turned to consoles.
As for the myth often used as an excuse for not developing for linux, that we won't pay for software because we use a free os, note which platform pays the most on average for the Humble Bundle It ain't Windows.
I actually bought parts, built a pc, bought and installed Windows, just to play games.