India, a third world country has worked it out. Apart from UPI, we have other bank transfer mechanism that doesn't involve writing a check. There's RTGS (Real Time Gross Settlement) for large payments. It's what we use when we buy property or do large business transactions. There's NEFT (National Electronic Fund Transfer) for instant transfers upto INR 500K. There's IMPS (The underlying mechanism of UPI) for instant transfers upto INR 100K.

Intra-bank transfers are always free and instant because bank is just changing numbers on two accounts in the same database.

It has been so easy to move money between banks in India that we don't even think about it.

A decentralized system like crypto which uses pointless proof-of-work to remove trust providing intermediaries are just too slow, expensive and overall inefficient by design. They won't work in real world.