A lot of older PC games can run just fine on modern phone hardware. I’d buy a SteamOS version of a phone that has some modular or built-in set of buttons and analog sticks. I don’t know how the app ecosystem would work for sensitive things like banking but it’s mostly a minor issue for me.
A Steam smartphone implies to me its main purpose is to play PC games and not caring about the other stuff. And no, for that reason I wouldn’t buy a Steam smartphone. Because playing games on mobile phone sucks, for controls (and I always need a controller in addition), battery life and size. And its probably as expensive as a PC. I would rather buy a dedicated handheld, if its available in this form. And if its based on Android, its a nogo anyway, for privacy reasons.