You should also keep in mind the big picture when it comes to decentralization. If the hard drives (or tapes) can only be produced by a small number of large companies like Western Digital or Seagate, then you can't really count those for a decentralized system. A truly decentralized system would have the devices needed to participate in (and verify) the system be easily created by a regular user of the system without relying on a central power. So for example, the hard drives needed to store the bitcoin transaction records should be able to be produced at a regular person's home on a 3D printer starting from just the raw materials. I don't know how close we are to this ideal, but just pointing out that it needs to be considered. This is also a reason why I like that Bitcoin uses the simple SHA sum for mining instead of a more complicated function such as scrypt. It makes it easier for small scale entities to understand and to produce the ASIC miners.