Basic question, if someone buys a game to run it through Proton do they look like a Windows or Linux user on your end?
How would you feel about games being made available for free to Linux users if it uses Proton. If the game contributes basically nothing to your bottom line and you don't plan to support them anyways, why not? It will only grow the audience more quickly.
Users running a game in proton show up as Linux users, not windows users. Because of this, many games already show a bunch of Linux players even though the game doesn't have a native port.
Couldn't Windows users boot into Linux then to get games for free? Like, it would probably increase the share of Linux users but Windows users would decrease. And when you start charging for the Linux port, a large amount of those users would flock back to Windows.
dual booting is a kinda a lot of work just for some free games.
You also could charge a dollar since you are often offering a strictly worse product for Linux users. That's what happens in many other markets when you are trying to expand a userbase.
There is clearly a dollar value to providing support. Why shouldn't the price reflect that?
How would you feel about games being made available for free to Linux users if it uses Proton. If the game contributes basically nothing to your bottom line and you don't plan to support them anyways, why not? It will only grow the audience more quickly.