Here's the difference: banks have every incentive to try to reduce their power consumption, Bitcoin miners have every incentive to increase their power consumption.
Not true. Since using more electricity costs more money, bitcoin miners have an incentive to use less electricity. Their incentives are to maximize hashes/s/watt.
This is true, but the reward is currently far larger than the electrical consumption, so mining is currently mostly a contest to bring more hardware online.
(kicking CPUs and GPUs out of the contest probably did a lot to lower power consumption, but I wonder if the growth of the network has overshadowed that)
Given that energy efficiency is mostly a function of the underlying silicon, miners don't have much control over it (especially once everyone has converged on 28 nm).
You are pretending that miners somehow do not double their income when they double the number of mining rigs they operate. A bitcoin miner makes more money by devoting more electrical power to Bitcoin, regardless of the hashes-per-second-per-watt ratio.
Increasing hashing only increases the proportion you get from subsidy + fees. It's a zero-sum game. If the other guy increases his hashing rate, you will lose out. Eventually, at some point, energy cost dominates.
OK, let's suppose that everyone is already at the point where consuming more power is not economical. Then a new technology that would allow everyone to use only 1/10th as much power for the same hash rate comes along. Power consumption will not decrease in that scenario; instead, it will remain the same, and the hash rate will increase.
My original point was that when a technology comes along that allows banks to be more energy efficient, the incentives are all in place for banks to adopt that technology and reduce their energy consumption. There is never an incentive for a Bitcoin miner to reduce their power consumption.
The reward for mining is currently 25 BTC/block, plus transaction fees. That's pretty high at today's conversion rates, but it will be cut in half continually over time. If miners spend more money in power and cooling than the reward, they're losing money. Even with deflation, the amount of money they can spend on power will go down.