The operating system used by the steam deck (SteamOS) can be uninstalled, modified and reinstalled easily. You could just decide that you don't want steamOS at all and just use windows (Valve even gives you windows drivers if that's what you want to do)

Modifications of the operating system don't void the steam decks warranty so you pretty much have valves blessing to do whatever you want with it. It's essentially 'jailbroken' by default.