Also satoshi didn't envisage mining like it is today. It is possible for the hashing power of all inefficient (uses more power than would gain in rewards) mining machines (and hence not connected to the network) to outnumber the hashing power of all mining machines on the network. This requires low block rewards in real terms, expensive electricity costs in real terms and a relatively small difference in efficiency between the inefficient and the efficient mining machines. In this case an attacker could gain 51% hashing power very easily  to perform a double spend since unused miners are virtually worthless (satoshi didn't envision this).

We need much more discussion about the security assumptions of bitcoin. New technologies may allow us to gain more security/decentralisation in some areas but at a potential trade off with other areas.