I agree there are cases where Kafka and MQTT are often used together. If you have lots of MQTT clients producing messages fanned-in to a small number of backend apps (or consuming a small number of wide fan out messages) people often combine Kafka as a fat pipe behind MQTT brokers (though there are alternatives, consuming messages from the brokers using e.g. MQTTv5 shared subs)
In more complicated situations (e.g. "outgoing" persistent messages buffered for individual clients (i.e. each client has a "message inbox"), Kafka is less obviously useful (it's an anti-pattern to try and random-seek the messages from Kafka topics as clients connect). In this kind of pattern, the main architecture pattern I see are clients partitioned across (highly available) MQTT brokers. If the messages come from MQTT clients directly to other clients (e.g. instant messaging (Facebook Messenger uses MQTT), having these broker in a cluster sharing a topic tree is very useful.
If the outgoing messages don't get buffered whilst the clients are off-line (e.g. because these are responses to client requests) then you don't need each client to be routed to a "home" broker that buffers the messages - it can connect anywhere.
It all depends on the shape of the message flow you're designing the system to support (but to say MQTT is a toy for small numbers of topics as it seemed to me that you argued up-thread seems misguided).
In more complicated situations (e.g. "outgoing" persistent messages buffered for individual clients (i.e. each client has a "message inbox"), Kafka is less obviously useful (it's an anti-pattern to try and random-seek the messages from Kafka topics as clients connect). In this kind of pattern, the main architecture pattern I see are clients partitioned across (highly available) MQTT brokers. If the messages come from MQTT clients directly to other clients (e.g. instant messaging (Facebook Messenger uses MQTT), having these broker in a cluster sharing a topic tree is very useful.
If the outgoing messages don't get buffered whilst the clients are off-line (e.g. because these are responses to client requests) then you don't need each client to be routed to a "home" broker that buffers the messages - it can connect anywhere.
It all depends on the shape of the message flow you're designing the system to support (but to say MQTT is a toy for small numbers of topics as it seemed to me that you argued up-thread seems misguided).