I have a desktop PC plugged into my TV, and have had for the past decade or so. The OG steam controller has saved me multiple times from having to reach behind to plug a keyboard and mouse. I understand you don't care for this use case, but the appeal of this controller is in steam Input, if you plan on using it just like a regular controller then you're better off buying an 8BitDo or similar for half the price.

There were open source alternatives for the OG controller, one that I used for a while is https://github.com/kozec/sc-controller and I'm sure similar ones will pop for the new one. Unlike most proprietary controllers each button and input sends a distinct input so a driver for it is trivial to write (most controllers with back paddles for example require some software to map them to other controller input and they send the same input from the button and the paddle making them impossible to work without the official software).

Could Valve have released an open source version of this? Yes, but they're selling the controller in their platform to be used in their platform so it's a weird ask, you already have to have a steam account to buy one, and SteamInput is a big part of why I use Steam even for non-steam games. And realistically without SteamInput this controller is not that appealing.