Dude, no. A Raspberry Pi 5 is less powerful than a Steam Deck across the board. The only reason it could run DOOM at 4k in that video is because the guy hooked it up to an external GPU, and that external GPU costs several times more that a new Raspberry Pi.

Also, the Raspberry Pi 5 doesn't come with an SSD. If you're going to build your own boxed similar to what you're proposing, you'd need to buy a Raspberry Pi + SSD + case + interface cards between the parts that'll cost an addition $50-60. You're looking at around $300 for parts. Any commercial product would also have to factor in labor and some margin for profits.

At this point you're looking at a product that costs about the same as a base-model Steam Deck that isn't portable and has less computing power for playing games. Virtually no one would buy that.

If Valve did release a non-portable PC, they might use a higher-power version of the Steam Deck's APU at similar price points to the Steam Deck, filling the space that used to be occupied by Intel NUCs. But whatever they do, a non-portable system from them cannot be weaker than a Steam Deck. It would be completely rejected by most customers on that alone. And it cannot be dependent on an external GPU since those are a waste of money compared to just building a PC.