Banks are very different from cryptocurrency. The economic value of banks is through loans, not monetary transfer. Through loans, and regulations/backing of a central bank, money is created. This economic effect is what enables economic growth. The money created by banks likely offsets the energy/overhead. https://en.wikipedia.org/wiki/Fractional-reserve_banking

Bitcoin cannot replace banks, even at full adoption we would need banks for loans, investments, financial planning, etc. Bitcoin could replace Visa, but it uses orders of magnitude more energy per transaction. So yes, it is an environmental disaster. I don't see any possible way that a decentralized system that is secured by doing useless computations could ever use less energy than a centralized one, it just doesn't make sense.