They do now for the Steam Deck, but customers can always choose to play on Desktop or x86 handhelds instead.

You can install other game stores on your steam deck. You can directly install apps outside of Steam. You can even install a different operating system if you want. It's not locked to Steam in the slightest.

Let me know when I can do install fdroid on iOS.

SD is ultimately a linux computer, so you can still throw on another store if you really care. It's also a relatiely niche device, so not quite prone to monopoly effects anyway.