Pretty much the entire (DIY) PC gaming market? Granted they may not buy Windows but they are certainly making the choice to download and use it over linux.
Sure there is a pre-built market for desktop PCs but I would think the PC Gaming market skews to DIY though I have no stats.
Note: I run arch btw.
Just a few weeks ago, I tried playing a Japanese game that I was interested in, but it didn't even get past the start menu. Not a big deal for me as I'm not an avid gamer, I just moved on.
But imagine telling a hardcore gamer that no, you can't play that new game released an hour ago. Maybe it'll become compatible a few months later, maybe not.
Or tell a professional artist that no, they can't use that art software anymore, they have to retrain their entire skillset with an entirely different software.
Now that's a show-stopper. For most people, it's 100% compatibility or bust.
Professional artists are also an interesting choice. If they are in a major art studio, they are probably having to use custom tooling there already. And Wacom is very well supported on linux.
I have a game that works fine with proton on one machine, but doesn't on another machine. But works fine with windows on both machines.
I tried for hours to get it to work on the other machine, since it clearly can. No luck.
In the end with proprietary software, it's always a black box.
This is also an odd rabbit hole to fight about. I've had so many windows boxes that couldn't play games throughout the years that I've basically accepted that games programming is hard. :D
No difference of being a nice package for something like MAME.