But that's the gist of the article here, right? That Postgres is taking over all db-like use cases. It doesn't claim that it can replace Kafka but https://www.amazingcto.com/postgres-for-everything/ certainly does. Of course it's not a full replacement, but it might be good enough.

If your premise is that Postgres is eating the datastore world, then you're talking about using it as a replacement for Kafka and Cassandra.

Frankly if you zoom out far enough they're all systems suitable for use as your primary online datastore that you build your application on (each with their own caveats of course). There are places where they compete.