Steam Deck is a computer, so its users deserve to have full control over it just like their PC or smartphone.
You are correct about Steam client though. Even if they keep the internals closed, the GUI part alone would be worth forking. I wish a chrome-less version would exist.
If people can't easily modify it, then its developers have power over users. You have to trust that they will not abuse that power, but they already do - with DRM for example.