People talk about environmental impact per transaction because a transaction is the "useful" part of Bitcoin (even the coins themselves do not really exist, the

People talk about environmental impact per transaction because a transaction is the "useful" part of Bitcoin (even the coins themselves do not really exist, they are actually the unspent outputs of transactions). Yes, since for now the block rewards are still much bigger than the transaction fees, the energy impact of miners is still not very correlated with the number of transactions; but you can still attribute all that cost to the "useful" work it did. That is: the transactions did not (directly) cause that cost, but they are the "benefits" of incurring that cost. It makes sense to think about how many "benefits" you got from a given amount of costs, or inversely, how much cost there was for each unit of "benefit". To put it another way, if the costs are identical, it's better to have 1000000 transactions than just 100 transactions; "environmental impact per transaction" (or its inverse, "transactions per unit of environmental impact") captures that perfectly.