I guess this is the case. Windows PC sales keep dropping as more and more people satisfy their everyday entertainment needs with smartphones and tablets. A quick game here, some browsing there, maybe a song or movie now and then... Windows are less needed day by day.
Call it a personal knack, but I think Valve felt this wind of change and thought they could use it to their advantage by issuing a new OS. If Steam OS was to come with every new PC (call me Steam Machine) then they could dethrone Windows.
With Windows 8 failing hard, situation is going to worsen.

This, I stole from grandpa...It's called soap.







