Here is a quick listing of the real limitations of hard drive RPM as found in a post by TheSSDGuy. The comments underneath the article held a wealth of information, which I've consolidated here.
It is not because the disc might break the speed of sound:
- A 3.5" disc requires 73,105 RPM to break the sound barrier. The real reason is power consumption, which increases disproportionately with speed.
- Multiple arms can't be used to increase throughput since at high speeds each arm causes turbuence that makes flying low hard.
- At high speeds disk substrates (metal or glass) would stretch or disentigrate.
- Higher speeds also magnify high-frequency flutter.
- It takes time to ramp up the charge on the write head. As the speed increases the capacity of the disc decreases (unless the write head charge can be sped up).
- Rotational delay is much less than seek delay, so wrong bottleneck.