I don't know, my personal opinion was that RethinkDB had its head on straight, MongoDB is garbage and still neither is so much better than PostgreSQL that I will switch away from it.
Postgres is the default datastore (because schemas are awesome) for me, and I haven't had a use case yet where I needed something that Postgres wouldn't do. Maybe if you have a very specific need, you'd reach for another datastore (come to think of it, I have successfully used Cassandra for multi-DC deployments of a distributed read-only store), but that's not the norm for me.
Postgres is the default datastore (because schemas are awesome) for me, and I haven't had a use case yet where I needed something that Postgres wouldn't do. Maybe if you have a very specific need, you'd reach for another datastore (come to think of it, I have successfully used Cassandra for multi-DC deployments of a distributed read-only store), but that's not the norm for me.