Also DirectX 11 was originally supposed to be the last major DirectX version with only some minor updates planned, since Microsoft saw no additional parts of the graphics pipeline that could additionally accelerated by hardware (Source: some AMD talk a few years ago).
A few months ago DirectX 12 was announced (and it's quite probable that AMD's Mantle was the reason for this roadmap change).
I imagine future versions will be Windows 10 II, Windows 10 III, Windows 10 IV, Windows 10 V, etc.