Is there a way to move Elasticsearch data into RDBMS?
Yes, you can do this through Logstash, using the JDBC plugin.
What I'm talking about is not "DB -> Elastic", but "Elastic -> DB".
As far as I know, there is no jdbc plug-in in Logstash as output.
As far as I know there is not anything available out-of-the-box, so I suspect you will need to create a custom application or script.
Right. There was a third party logstash-output-jdbc plugin, but it was archived by the owner.