Redis does a lot more than just store keys and values. Functionally speaking Redis Pub/Sub and Kafka are interchangable up to a certain level of throughput.
They really serve very different use cases. Redis pub/sub consumers only receive messages while connected, whereas Kafka consumers can pick up where they left off. Event ordering, back pressure, etc.
There are many use cases Redis pub/sub can't serve beyond just scalability.
Kafka is a distributed, fault-tolerant and highly scalable message broker.
Redis is a very fast key/value (another other data types) store.
I suppose that at a high level Logstash can be compared to Kafka but IME Logstash can't handle scale. It's trivially easy to bring Logstash to its knees.
Elasticsearch is, well, a search engine.