Counter argument: 1 postgres instance is far easier to manage than a slew of servers. I can spin up a heroku environment with a postgres very quickly, and with little effort.

If you're starting out with a team of say 4, and that team will stay small, don't waste your time integrating complex tech.

Solr is DEFINITELY better than postgres fulltext search. However pg_search and a clever index has let me solve 95% of all our search needs in almost no time with zero maintenance. However once you want to get into search complexities, Solr quickly outperforms postgres, but you have a lot of setup and env management to do now.

So it is all about team size. My instinct is at around 10 engineers you should start thinking about which complexity is worth pulling out of postgres and into its own service based on needs.