Rotational storage cannot keep up with the new kids on the block - flash memory.
For years people have forecast the demise of rotational storage. The argument is that the price-performance of faster solid-state memory will also continue to improve, and that one day rotational technology will cease to improve as quickly. Part of this argument is based on the fact that disk drives include moving parts, which are inherently inefficient.
This view was not uniformly held, because rotational storage kept improving so much, staying a step ahead of solid-state. But now it seems it may be correct.