Blockchain is only potentially useful if there’s no single entity that can be trusted. If banks can’t even trust themselves to manage their own internal ledgers, they have much bigger problems to deal with.

Blockchain has been around as a technology for nearly two decades. If financial institutions thought it could help them you can bet they would be all-in on it by now. As it is, blockchain has no significant advantages over traditional financial ledger systems, so what incentive is there for them to use it.

It's not something new or cutting edge any more, just waiting for a bright spark to discover the technology and put it to use.

How is the blockchain different from a read only ( write only once to be specific) DB that follows ACID?

Why would you want the computational power of a bank system have anything to do with whether it's ledger is correct?

Yeah let’s use the computing power of an entire country to pay for a small coffee.