Another factor is how many ordered streams you can have. Typically a few thousand at most with those systems. We take the serverless spirit of S3 here, when did you have to worry about the number of objects in a bucket?
We are also able to offer latency comparable to disk-based streaming like Confluent's Kora and Kinesis, with our Express storage class (under 50 milliseconds end-to-end latency for client in the same cloud region) - while being backed by S3 with regional durability! Not a disk in the system.
We want people to be able to build safe distributed data systems on top of S2, so we also allow concurrency control mechanisms on the stream like fencing. Kafka or Kinesis won't let you do that. This is the approach AWS takes internally (https://brooker.co.za/blog/2024/04/25/memorydb.html), but they don't have that as a service. We want to democratize the pattern.
ED: on throughtputs, to clarify, I am talking about ordered throughput, i.e. per Kafka partition or Kinesis shard. WarpStream also does well here because of their architectural approach to separate ordering, but at a latency cost.
