But the PC hardware market has been in decline for longer. Casual users, and even some power users, are making the switch to laptops and tablets.
Also Steam Machines will be produced by a number of different manufacturers (many of whom were at CES 2014), so you'd have to combine the figures from the 13 different manufacturing partners. But this ignores that the OS is available for those who want to build their own Steam Machine. So it's likely we'll never know, unless Valve release figures showing the number of Steam OS devices.