I am happy about the raise of popularity of the new handheld PC consoles, but still Windows on handheld device is a deal breaker for me.
I know Steam Deck is not perfect, but the proton compatibility layer, the OS and the console-like experience is something I would pay extra money for, if other manufacturers adopted it.
On the contrary, you should be paying less, since you don’t have to pay for a Windows 11 license. And while Steam can charge whatever they want for their Steam OS license, it shouldn’t be as much since they haven’t had to develop a full fledged OS.
Windows, ad delivered by OEM like Lenovo and Asus, comes with additional third party bloat/spyware that eat resources but gives extra money to the OEM. These money are used by OEM to either cut the price of the windows license key or cut the overall price to the customers if Microsoft has already a special agreement with the OEM to provide free keys.
My understanding is that because it's Linux they can customize it and trim all the fat to make the OS run as efficiently as possible, making it perfect for a portable device where you want as much processing power going to the game. You just can't get that with Windows.
SteamOS is almost entirely open source software, except for the handheld's specific proprietary drivers and Steam itself. Vendors are free to use it via its open source license if they choose.
The hardest parts (i.e. proton) are fully available to anyone who'd like to use it under an approximation of the MIT license, even for commercial use.
You can install wine or steam and run games on pretty much any distro. SteamOS is just tailored for the Steam Deck and is open source and under GPLv2, so anyone can fork it or contribute (https://github.com/ValveSoftware/steamos_kernel).