The point is in general for people to just consider it, often people start out on their side projects or internal company projects and commission Elastic, Redis, Postgres, Kafka before even getting started. In reality they could fit it all into Postgres for a very long time.

Nobody is saying that a huge ecommerce store with complicated filtered search logic should throw away their Elasticsearch cluster and switch to Postgres.

If you actually start looking into these things, you often start looking at custom pg extensions, which means you just made the decision to "simplify" your stack by maintaining your own postgres cluster with custom extensions. This is just papering over the fact that you're increasing the complexity and saying "well it's still just postgres!" as you do it.