Another reason Elasticsearch is a better primary store is that Snapshots are Consistent at a time point. This is a natural benefit of Lucene's immutable files. Cassandra backups are Eventually Consistent and are difficult to restore. I saw co-workers doing a Cassandra restore; it was not a good day for them.
For complete disaster recovery, restore a Snapshot from time T, then have Logstash consume messages after time T from Kafka. This will result in reading some messages twice. If the application assigns document ids (or Logstash computes them from other fields), then indexing requests will just replace a duplicate in Elasticsearch.
If a shard goes Red and can not be restored, this disaster recovery method will work well.
As a distributed primary store for Time-Series Event data, nothing is more solid than Elasticsearch using these techniques.