Are you still using kafka? Does Kinesis Firehose read from Kafka?
At that point, might as well set up a Kafka Connect cluster with a Kafka Connect Sink, and dump to S3 yourself.
But you're right, no reason not to start with kinesis in the first place, especially with all the AWS options around it including Kinesis Analytics (Streaming "SQL"), Flink (supported by AWS now) or plain old EMR (Spark, Flink, etc...).
There is a lot of work that goes into maintaining your own Kafka ecosystem, even if you use Confluent, Lenses, etc...
Kinesis Firehose doesn't read from Kafka, we switched our code to publish to Kinesis instead.