I agree a robust, reliable db is super important - but isn't Mongo too, at this point? Even if it wasn't back in 2012? I thought most of the reliability issues and bad defaults were fixed by now...
In my planning for problem domain changing, personally I'm going to be writing a (relational) schema for the new data model anyway, which will be Postgres, and playing my event store over it and doing one-way writes to it going forward anyway. I'd rather reduce the mental maintenance overhead of two different databases to one.
If you're building your relational db based on the event store, I'd imagine even something simpler like Mongo would do it, but I agree that having a single db to maintain is an important advantage.