UPI can potentially be scaled to all countries. I think the biggest USP of UPI is the interoperability between payment apps(Google Pay, Paytm, PhonePe, etc), VPAs(alice@icici, bob@axis, dan@upi, 9848123456@paytm) and banks. One could have an account with bank A, create a VPA with bank B and use that VPA to pay a business through virtually any of the payment apps(GPay, Paytm, PhonePe, BHIM, etc). The main pain point UPI addresses is instant transfer, zero cost transfer irrespective of transaction size and removal of friction due to credential exchange. NPCI (the creators of UPI) are already in talks with the UAE and other neighbouring countries to build a cross nation interoperable payment stack. Since no player in the entire stack can charge a service fee, the way that payment apps have monetized UPI is by acquiring customers through payments and selling value added services like insurance, prepaid mobile recharges and mutual funds/ETFs. Interoperability allows for balancing and decentralisation of network load across multiple providers, making transactions less likely to fail. Also cross border exchange is another problem that is still unsolved. Maybe a UPI backed approach to real-time currency transfers is a pipe dream but in a ideal world every transaction, irrespective of point of origin, size or currency would be as seamless as an UPI transaction.

On a side note, I wonder if something like this could be built in the web3 space. A totally decentralised public blockchain implementation of the payment network using UPI-esque interoperability between payment providers and banks.