Basically because people end up engineering their microservices as a shim to funnel data into the “magical black hole”

From my experience most microservices aren’t engineered to handle back pressure. If there is a sudden upsurge in traffic or data the Kafka cluster is expected to absorb all of the throughput. If the cluster starts having IO issues then literally everything in your “distributed” application is now slowly failing until the consumers/brokers can catch up.