[miguno]

Both Kafka and Pulsar have this kind of bottleneck in your scenario -- say, one "hot" write partition.

If one Kafka broker or BookKeeper bookie node cannot keep up with the write load (e.g. network or disk too slow, CPU util too high), you must add more partitions. For Kafka, for the reasons you already mentioned. For BookKeeper (and Pulsar), because only a single ledger of a topic-partition is open for writes at any given time.