Density play only
There's no room to remove cost from a drive. We are not going to see any major component disappear form the parts list. The only play for the drive makers is density, but here they are slowing down as the technical barriers for each step get greater.
Hard drives can't get faster, and they are already way slow compared with SSD. Add to that, SSD are lower power, silent and robust.
The demise of the hard drive is inevitable, just as PCs will be replaced by mobiles and tablets. It's happening faster than WD or Seagate would like.