My old Ryzen 3700X gaming PC has 16GB of RAM and 8GB of VRAM (RTX 2070 Super) and there isn't any game that runs better on it than my Xbox Series X. And the GPU in the Steam Machine is slightly worse than an RTX 2070 Super.
You typically don't want to do this anyway in games. You're probably doing something wrong if you're reading textures/meshes on both the CPU and GPU.
> Don't forget about how much more RAM a general purpose OS like Steam OS can consume versus a gaming specific OS too.
SteamOS is meant to be a gaming specific OS first. It has a desktop environment but none of that loads unless you switch to desktop mode. That's just taking up some disk space while you play games.
I think it's fair for some of us to consider the resource usage of a core feature and not really accept "just don't use desktop mode" as a viable suggestion. Especially if half the pitch is "it's a mobile PC." You can't use many of the features it's capable of in gaming mode.
It is trivial to switch between gaming mode and desktop mode. I'm not sure what point you are trying to make here.