But how many billions of transactions are these networks handling? BTC is using vastly more power per transaction. What would be the power usage of BTC if it were scaled up to support an actual useful number of transactions?
I am getting tired of seeing this point being raised time and time again. But I am going to assume that you are just uninformed and try it one more time. There is no meaningful 'cost per transaction' in Bitcoin. There could be 0 transactions per block as well as could be > 100 millions transactions per block (assuming people are using L2 solutions). The cost is used to sustain the whole network.
That's like saying "There is no meaningful 'miles per tank of gas' for cars. You could spend the whole tank idling or you could go across the Pacific on a single tank (assuming you're being transported on a ferry)."