The moat
Global acceptance network shared with the card duopoly. Brand, issuer relationships, and fraud/data services. Efficient scale: two networks is the stable end-state in most markets.
Pain, risk, or retraining required to leave.
The product gets better as more people use it.
Patents, data, licenses, process knowledge, regulation.
Fixed costs spread over a volume rivals cannot match.
Structural ability to be the low-cost producer.
Pricing power from trust, habit, or identity.