It seems spindle speeds are getting slower. When was the last time someone advertised "New now 40% slower!". Much of this has been attributed to the desire for lower powered drives. The problem is that there aren't many benefits to a lower powered drive. I think people confuse low power with low energy, which are two different things. Low power means a small amount of juice consumed at a given amount of time, but what's really important as far as battery life or the environment is concerned is low energy. Energy takes into account not only the juice drawn but the amount of time you are drawing it for. Therefore a low power drive isn't going to save juice if it has to run for longer to get the operation done.
Thus the ideal solution is not to use lower spindle speed drives but to use variable speed drives that can be configured in software as needed. One could set their drive to discrete setting like 5400RPM, 7200RPM, 10kRPM or 15kRPM or auto for instance. Or maybe anywhere in between.
If Western Digital is obsessed with price discrimination then it could lock the 10k and 15k speeds and charge users for an unlock code. But it wouldn't have to manufacture separate sku's for these drives which is a great thing for WD.