Understand Lucene to Understand ElasticSearch
medium.com
medium.com
Every tool has its place. Postgres works wonderfully in 90% of user-joe cases, but when you start to shovel in TBs of data, you'll need to look for something else.
Maybe eventually RDBMS systems will have full-text search out of the box that makes it unnecessary, but until they do, any predictions of death are certainly premature. (also, the lifecycle of indexed documents is different enough from transactional records that it's not as easy as it sounds).