> I've often heard wisdom like "it's irresponsible to use Kafka when you could just use RabbitMQ" but why does that statement not work equally well in the opposite direction?

It could, in some circumstances. Generally, though, the two have only a very small overlap with respect to messaging use cases. One was designed to be general purpose and solve a variety of MQ use cases and serve even "dumb" clients. One was designed very specifically to impose a high burden of state tracking on clients for a very specific paradigm.

> It seems like some "you don't need this fad technology" sentiments don't provide a whole lot of justification for sticking with the unfashionable choice.

The "unfashionable choice" that works doesn't need justification. The new shiny one does.