> Why not just run ElasticSearch on a single instance then ?
Sorry, I am not sure why you brought up a single instance. If you run on a single instance, you lose half of the benefits of ES which is trying to be highly available.
If we have an index which does not need any backups, single instance is fine, but for most people single instance is not an option. Furthermore, if one is already struggling with 3-4 nodes, how helpful is single instance?