One more thing to add. Consider these scenarios:
We have approx. 14 million coins left to mine.
1) If the price of bitcoin stayed at $10 tomorrow then by the time all bitcoins are mined we will have spent in the region of $140 million dollars on electricity.
2) If the price of bitcoin rose and held at $100 tomorrow as a collective group we would spend $1.4 billion dollars.
What is gained in the 2nd scenario? The security provided is surplus to requirements and cannot exist at that level when the 50 BTC block rewards stop. We are paying to distribute wealth but using that wealth in the process.
The need for security grows together with the overall market cap of the system which grows with the price of bitcoin - so this is actually a correct result, this additional security is needed even if the number of transactions stays the same. I've argued that if the system grows and has more transactions then probably the market cap will also grow - but this is not direct relation.
Interesting, i had only thought of increased transaction volume requiring increased security, but I guess the amount available to steal i.e. the market cap is the real driving factor.
$1.4 billion dollars on security seems like an expensive requirement for something only worth $2.1 billion. The point still stands, as the block rewards decrease the security of the system will fall, so at least some must be surplus. Really its better for the collective group to not push up the market price until enough transactional volume is being processed, else we are just wasting extortionate amounts of money on not much.