Often, yes. If your "Kafka use case" is event distribution between services, work queues, short-to-medium replay windows for recovery, and modest sustained throughput — which describes most microservice estates honestly — JetStream covers it with at-least-once delivery, durable consumers, and replay from any point in the stream. You also pick up things Kafka never gives you: request/reply, wildcard subscriptions, a built-in KV store, and an operational footprint small enough that the messaging layer stops being a team’s job.
The cases where it cannot replace Kafka cluster around two themes. First, the log as system of record: indefinite retention with tiered storage, compacted topics feeding caches, replaying years of events to bootstrap a new consumer. JetStream can retain a lot, but the design centre is messaging with memory, not a permanent database of events. Second, the ecosystem: if your architecture leans on Kafka Connect’s connector catalogue, Debezium CDC, Kafka Streams or Flink jobs with exactly-once processing, ksqlDB, or schema-registry governance, there is no NATS equivalent for any of it — you would be rebuilding a platform out of application code.
A useful tiebreaker: count the systems that consume your streams without you writing code (warehouse sinks, search indexers, CDC sources). If that number is high, the connector ecosystem alone justifies Kafka. If it is zero and the consumers are all your own services, you are probably paying Kafka’s operational tax for shape you do not use — exactly the situation NATS was built for.