The main thing to compete on is distributed scale-out and high-availability which PostgreSQL does not have a good answer for.
A huge proportion of modern software businesses can run on single-writer RDBMS instances, properly engineered, at a fraction of the operational and implementation cost of a scale-out solution. That applies to hosted and self-managed solutions equally, in my experience.
Modern distributed databases scale better but also have better replication, high-availability with automatic failover and no downtime, easier upgrades, easier backups, and generally less maintenance. Removing the single-point of failure with efficient distribution while being able to run easily on docker/kubernetes makes a big difference over a single monolithic database server.
You might also end up with a LOT of servers unless you starting sharing servers for different shards (but still with different sets of 3 servers for every shard).
Overall, I agree -- PG with JsonB is extraordinary powerful system, covering document oriented and typical row-oriented use cases.
PGs ecosystem with external foreign interfaces, and in memory streaming solutions (like PipelineDB) - must be an envy of any other db ecosystem.