My concern is the energy cost of running the network. We can discuss that cost in terms of how much energy is used to confirm each transaction by dividing the e

My concern is the energy cost of running the network. We can discuss that cost in terms of how much energy is used to confirm each transaction by dividing the energy cost of mining six blocks by the number of transactions in a block. Letting us communicate in terms of the energy cost to confirm a transaction allows us to compare the efficiency of Bitcoin to other payment systems. You're right: the number of transactions in each block is not relevant to the amount of energy is used to mine that block. The network could decide to include more transactions in a block. Some Bitcoin forks tried doing exactly that, actually. Right now it's 500, but if enough miners agree tomorrow it could be 1000. All it does is change the number in the denominator of the energy cost per transaction calculation, but it won't change the fundamental incentive structure of the network.