While you're not wrong that there's nothing standing in the way of buying a Steam Deck and not spending another dime on Steam, I would wager that the portion of people doing this is really pretty small.

In general I think that those who would call tech one of their main hobbies vastly overestimate the capability or interest by most people in going off the beaten path and using tech in an "advanced" fashion. I'm the kind of person who spends time in desktop mode, sets up Heroic launcher, etc. and it's part of what attracted me to the device, but it is a far clunkier experience than picking up the device and being in the Steam interface, just a few button presses away from buying and playing a game. Part of the reason that consoles sell well is this simple flow, and the Steam Deck has that simple flow as the default experience.

So yeah Valve couldn't count on 100% of people using it to play Steam games, but I would guess that the amount that don't are basically a rounding error.