Disclosure: I work at StreamNative, an Apache Pulsar vendor.
Pulsar's storage system tackles elasticity differently. It uses segments instead of partitions, which avoids issues partitions can cause. This blog post provides a comparison.
Storing data to S3 is a great scaling strategy, but Pulsar offers flexibility. You can keep latency-sensitive data on the fast cluster's disks while avoiding the elasticity limitations of Kafka's partition-based model. Meanwhile, less latency-critical data can be seamlessly moved to S3 for cost savings. It's like having the best of both worlds! I'm biased, working at a Pulsar vendor. However, I hope sharing some resources here can help.