I mean, it's not like there is a literal $150B pile of cash sitting in a vault anywhere. In practice "sitting in cash" means holding short-term Treasury bills / bonds, i.e. effectively loaning that money to the government to pay for public expenses.
The computing power used for proof-of-work is not proportional to the number of transactions. Computationally, it's relatively cheap to verify the entire transaction history.
The real energy consumption comes from miners who use a crazy amount of computer power to secure the network. Bitcoin's transaction history is immutable because changing it would require an even crazier amount of computing power - too much for any malicious actor who tries to attack the network.
The amount of energy used is a feature, not a bug. There are no alternative designs known that could replace proof-of-work while keeping its security assurances, permissionless participation, fair distribution of coins, unforgeability, etc.
If you only look at the energy cost of running the servers where the banks store their databases, sure. But what about the energy cost of powering air-conditioned and brightly lit office buildings all around the world, thousands of branches and ATMs, security systems such as secure vaults and armored trucks, and the millions of employees commuting every day (or in 2020, Zooming into meetings all day)? It adds up real quick.
The difference is that the Bitcoin network is completely transparent and auditable, and we can accurately estimate the total energy used to secure it; whereas the banking network is opaque and we can only guesstimate how much energy they really consume
Securing the integrity of a monetary network that coordinates economic activity at a global level is not a "ridiculous use of power". Christmas lights are a ridiculous use of power
How is the underlying trust mechanism inefficient? You can run a fully validating Bitcoin node on an old laptop without any issues. Even a raspberry pi can handle it.
This concern usually stems from the idea that the energy used by the Bitcoin network could be otherwise used for more productive functions. But if, like many, you believe that bitcoin can succeed in becoming a decentralized world currency, the reality is that there may be no greater, more important use of energy than to secure the integrity of a monetary network which coordinates economic activity at a global level.
I highly recommend this piece (which I'm paraphrasing here) if you're interested in the topic:
> Maybe charge people to use the app?
Who will pay for a service that's offered for free by at least 3 major messaging apps?