Gamescope does a lot more than scaling. HDR, better framerate limiting, non-rgb gamut handling, isolating "fullscreen" games, shader effect loading, and probably more I'm forgetting. Flatpak let's you run the Steam app itself correctly, nothing more. These "you only need to fiddle when" are exactly what add up and create frustration between users on different systems just wanting to play a game instead of read a manual/guide to find which "few tweaks" they each need to do to get the same experience as SteamOS would give them.
HDR will work soon work everywhere on Wayland by default.
You seem to think steam on Desktop Linux is somehow different from steam on steamdeck Linux. Like I said in my previous post, the only "fiddling" you need to do is copy pasting launch commands and choosing a different Proton version from a drop down _just_like_on_steamdeck.
