A BTC transaction (on chain) brings the miner about $100, but most of that is from the coinbase mining reward, and a small part only from transaction fees (thou

A BTC transaction (on chain) brings the miner about $100, but most of that is from the coinbase mining reward, and a small part only from transaction fees (though that proportion will grow, presumably, as coinbase halves to zero). A good chunk of that will be energy costs, so an on-chain transaction will have significant energy costs (and probably way more than the current average fee of $2.65/tx, which, btw, is more than your "email transfer"). Off-chain transactions (like lightning, or from one exchange account to another at the same exchange, or from one account to another at the Salvadorean "wallet") can of course be quick and cheap, like any centralised banking.