The miners must set a rough minimum fee though. If energy costs rise enough at some point the miners will need to slow/shut down unless the sum value of the fees (in whatever currency is used to pay for the energy) rise above some threshold.
I'm not even considering the block reward, consider when there are only fees (or block reward is negligible). In that case profitability is determined by energy/hardware costs vs fees.
If costs go up (eg energy becomes scarce) the minimum fee also need to rise. Each "unit" of economic activity using the currency will be more expensive when energy is scarce than when it is abundant. This is a natural feedback loop.
If costs go up, mining activity drops. Miners can not increase transaction fees. All they can do is accept the fee they're offered or not.
If they're mining blocks at all, they want to accept the maximum fee that is offered. If they say "you guys aren't paying enough, I'm going to stop mining", then other miners will just get the fees that people are offering instead, and when the difficulty adjusts downwards, the miners that stopped mining might start mining again.
You seem hung up on the fact that miners do not broadcast a fee as part of the protocol. Ok, nobody is arguing with that in this thread...
It is just like I will not pay $2000 for a cheeseburger. I am not going to advertise that, I simply will not buy it. There is some maximum price that I set for myself.
However, the role of difficulty adjustments is a better point. As energy costs rise there could be two (non mutually exclusive) effects:
1) Fewer transactions are made with higher fees
2) Mining slows down, leading to a difficulty decrease, leading to less secure transactions
Less secure transactions means waiting for more confirmations to get the same level of confidence there will be no double spend. Is this not another, less extreme, way to slow economic activity in the face of increasing energy costs?
