I don't think reducing the price of Windows for OEMs will do much as the SteamDeck uses Arch Linux specifically for better control of the software/updates than

I don't think reducing the price of Windows for OEMs will do much as the SteamDeck uses Arch Linux specifically for better control of the software/updates than relying on Microsoft. They can try making Proton's job more difficult, but I'd expect that major changes to the APIs would prevent a lot of existing games from working on Windows. Legally, I don't think they've got a leg to stand on.