For cloud-native applications, Kubernetes and Istio deliver a lot of important functionality out of the box, like ensuring resiliency and scalability. This functionality works generically for microservices, no matter in which language they have been implemented and independent from the application logic.
Some cloud-native functionality, like application-specific failover functionality, metrics, and fine-grained authorization, cannot be handled by Kubernetes and Istio, since it needs to be handled in the business logic of the microservices.