Energy spent on mining could also reduce to match the fees on offer. It doesn’t have to just be fees increasing.
Discussion
Correct, but if energy reduces, then its easier to have 51% hash power and compromise security.
Of course you can say that, when that time comes, no one will want to damage the network, BUT the risk exists and will be higher.
What can happen, is that the price of energy in terms of Bitcoin reduces, and thus the fees can increase proportionally to the decrease of block reward, keeping the system more or less as is. But this means that energy must be valued in Bitcoin and that energy prices will fall gradually in terms of Bitcoin.