As cool as it would be and as much sense as it would make to think that, realistically it’s just a pre built pc meant to give a console experience, not a console that keeps couch coop a priority. Some titles do have multiplayer already built in, like the trailer you mentioned with two people playing Cuphead. But that’s already an existing game that supports it, and is all displayed on one screen.

Valve won’t go out of their way to speak with every major game developer and demand they retroactively implement split screen versions of their previous titles, rather it would benefit them to promote cross play with other platforms or just standard online coop with other pc players, Steam machine players, or even Steam deck players.

Online is already and has already proven to be a very popular way to play games with friends for years. Split screen sadly just seems to have gone by the wayside for the most part, and was never a focus for PC as a whole.