Imagine a Steam TV with the Steam Box simply built-in. That would be incredibly nice. The worst part of my brand new LG G5 OLED TV is the software itself. I'd pay a good deal more to have Valve responsible for the software running on my TV.
There's actually a quasi-standard of TV-compute unit interface made for industrial displays. This could be really nice for things like steam cards that could just slot into TVs with whatever performance you need.
And even better make it as open as Steam Deck/Machine and allow to install any GNU/Linux distribution onto it maybe even something with KDE Plasma Bigscreen or something similar if desired.
You can get TVs with a "PC slot" like the Sharp M431-2. Just need a Steam Slot.
It might be nice for a little while, but the PC component is going to age much more poorly than the display will.
I think the better move would be for Valve to make a really nice gamer-oriented dumb TV that's essentially a 50"+ monitor. Kind of like those BFGDs (Big Format Gaming Displays) sans the exorbitant prices. The size of a Steam Box is in comparison quite diminutive, so finding a place to put it shouldn't be too much of an issue and the ability to swap it out for a newer model with the same screen 5+ years down the road would be nice.