Note that both use cases (decentralized global currency and smart contracts) rely on altruism which cannot be guaranteed by human actors but is entirely deterministic within the chaos of humanity (with a sufficient number of honest actors) using a blockchain. That is the value prop: distributed consensus with "quick" convergence toward an agreed upon truth.

Transactions between banks need clearing and settling. Many systems run on a periodict (like half an hour) net settlement, some are even slower. Blockchain is a perfect firt for those usecases.

There is a reason JP morgan is developing quorum

There are two distinct classes of system pushed under the label “blockchain”: the kind like Bitcoin are enormously complicated by trying to be used between untrusted parties. The systems which are potential viable are much less expensive because all of the parties in their system are known and trusted at least to be subject to contract law. All a bank needs is public key encryption and Merkle trees, and while that might be branded as a blockchain for marketing purposes that doesn’t make it the same.

Or they could just start doing real time settlement, something which the banks are working on.