Then it arguably isn't that either. They give you full instructions on how to repair and upgrade it, and they partnered with iFixIt. People have modded in more storage, battery life, and better screens. Personally, I think I draw the line at the part where it runs the same executables as any other PC, so I'll call it a PC.

Wow, it's been 40 years since I played on a spectrum.

Those are two different things

being a self-contained wad of hardware

Steam Deck checks this, but so do laptops, raspberry pis and smartphones.

unable to be upgraded or repaired piece by piece.

Again Steam Deck is almost as upgradable and repairable as a laptop, and more repairable than a raspberry pi or a smartphone.

So that definition of console doesn't work, otherwise raspberry pies, laptops, and especially phones would also be consoles. The differentiating factor is locking of the system with the hardware, in that sense Apple is more "console-like" than non-Apple competitors. Also The primary function of a gaming console must be gaming.

With those two extra points the Steam Deck hits one but misses the other. It is primarily for gaming, but the system is not locked down, you can change it how you want and even remove it entirely and put a different one.

So with any definition you can find the Steam Deck is not quite a console, but it does provide a console experience, so it's in a weird space.

I upgraded my Steamdeck joysticks to a 3rd party with hall effect sensors, the ssd to one with double the capacity, and the fan to one that is silent. There's people that have upgraded even more things, to the point of using a pcie flat cable to connect a full pcie GPU card.