That still doesn't explain margins though. Competing processors keep pace with fraud-fighting tech as well. For large customers, processing is still a commodity

That still doesn't explain margins though. Competing processors keep pace with fraud-fighting tech as well. For large customers, processing is still a commodity service and they have a lot of negotiating power to call up Stripe and demand they lower their rates. Until we see Stripe's financials I would not assume anything about healthy margins. Square also had a lot of hype in the early days but turns out their processing margins were really low and most of their current valuation is from expanding into other products like business loans. Stripe's valuation is similarly based on a hypey "grand vision" to offer other services that actually make money, not by magically making businesses pay more for commodity services.