MQTT and Kafka solve different problems. At my current company, we use both.

Kafka isn’t a queue. It’s overkill to use it as one.

Kafka is a great place to persist data for minutes, hours or days before it’s processed. It fully decouples producers and consumers. It’s also stupidly complex and very hard to operate reliably in an HA configuration.

MQTT is good for when data needs to leave or enter your cloud, but persistence is bolted on (at least it is in mosquitto), so a crash means lost data even though you got a PUBACK.