Your first problem is use of the term "require."
We do not "require" the current hashrate to secure the network. The hashrate is a byproduct of miners wishing to get bitcoins.
This is true
now. The current causality is:
Inflation schedule set in stone -> Total revenue to be made from mining -> Network hashrate in equilibrium
But in the future, when inflation is negligible and we only have tx fees, it will look more like
Security is required -> Protocol rules make transactions scarce -> Fees are paid for txs -> Total revenue to be made from mining -> Network hashrate in equilibrium
So the need for security will, by one mechanism or another, affect the network energy usage.
I (honestly) don't understand how the transition from mining-for-bitcoins to mining-for-tx_fees will play out. Sorta' filed it away under "not the time to think about it, will cross that bridge when we get to it." Can anyone walk me through a scenario which results in a responsive & secure network?