ING's mobile banking backend team migrated from Java to Kotlin to power payment processing for 6 million users handling 4.5 billion transactions annually. The transition delivered improved code quality through null safety, better resource efficiency via coroutines for IO-heavy workloads, and enhanced stability during production migration. The team adopted hexagonal architecture, grew to 12 engineers with 10 writing production Kotlin, and established an internal community of 200 members. Kotlin's low learning curve, compile-time null guarantees, and seamless Spring Boot integration enabled the successful refactor while maintaining system reliability.