New Delhi, Sept. 22 -- Like every major policy move, UPI's new merchant discount rate framework has already drawn supporters and opponents. The debate is predictable: supporters see it as necessary for the ecosystem's health; critics worry about the cost to merchants and consumers.

But pause for a moment. Ten years ago, when UPI launched, it seemed almost too good to be true: an instant, free digital payment system built on open infrastructure. A decade on, it still feels that way. That is the genius of the model. But can it remain sustainable in the long run?

The scale of the system is already testing its infrastructure. UPI processed 23.66 billion transactions in July alone, a record. In April, the Reserve Bank of India mandated stric...