PostgreSQL does not have built-in clustering but Patroni does it automatically.

MySQL Group Replication isn‘t automatic in all cases, like starting after all nodes failed, and it has some limitations, but it is built-in.

Not saying that MongoDB ReplicaSet is bad, has been working very well for us AFAICT.

If you rely on Postgres and/or Mysql replication then you'll need a DBA on call. Lots of manual work involved in babysitting them when stuff goes south.

Mongodb is entirely automatic, you just delete bad servers and reprovision them automatically and everything "just works" with no interruption.

mongodb is not open

I disagree with that. The Server Side Public License is more open than the AGPL.

In the same sense that the GPL is more open than the MIT license; more viral requirements for openness are generally a good thing. I don't want Amazon and the ilk deploying hosted Mongodb clusters.

People have the most varied creeds and faiths. However the facts are that it doesn't respect freedom 0.

Freedom of what? Freedom of business for cloud providers? I don't give a shit about that.

Open source for me is a hedge against risks. (Especially political and/or legal ones.) Mongodb's current license resolves that issue for me perfectly.

Agreed. It seems like something really bad is going on. I hope the Mongodb security team will handle it ASAP.