You can still buy prebuilt though and slap SteamOS on it and youre there.

Dont get me wrong this looks very a nice product, but its nothing revolutionary.

This thing is meant for a living room media center. A prebuilt PC with discrete GPU is a much bigger profile (and probably cost). You could say, fine, go buy a small Mini PC. But a system with the current best AMD Strix 890m GPU not only is expensive at $700-1000, but would only have half the performance of the Steam Box if its conjectured performance is similar to an RX 7600.

The hardware is not, but the implications are pretty close (major gaming company is pushing a first party product of open hardware + open software with a linux box). It is literally the year of linux desktop.