You've made some good points there, I admit.
That's why Steam has Big Picture Mode, why steam machines exist, and why Steam Link was created.
But the thing is, consoles do BPM BETTER than steam does and have been doing so for YEARS.
A windows Steam machine basically has all the drawbacks of Windows, printer queues, system services, Firewalls, antivirus/antimalware etc etc.
A SteamOS machine is still Linux. For example Steam has built in game streaming on windows, but that doesn't work on Linux/SteamOS. If you want to gamestream on SteamOS it is back to the command line you go to fiddle with ffmpeg x11grab scripts or install OBS.
The Steam controller already makes life easier for many keyboard + mouse games.
Considering I first hooked up a keyboard and mouse to a console in 2002...the Steam controller is a solution in search of a problem that isn't really a problem. Which could describe Steam boxes as a whole.