Fewer moving parts means you move faster. More importantly, you can make architectural changes faster. When your infrastructure is basically “one Postgres instance,” new developers can get the full stack running on their laptop in minutes, not days.

JSONB columns for schemaless key-value store.
Job queues with a SQL table running SELECT ... FOR UPDATE SKIP LOCKED
Full-text search built-in

Yes, you should use the best tool for each job, but every tool is another thing that can break, to deploy, to hires need to learn and to keep up.

When to not use Postgres: the queries starts taking 100s milliseconds and the indexing is exhausted. Once this happen, you'll have a clear spec