I think you could turn JSON into XML with a little work, and then run xslt on it in the db ;-)

Don't worry I won't do it really but does anyone worry that Postgres is doing too much and that focus might be lost on being a reliable and fast relational DB?

Not really. PostgreSQL has never been a purely relational DB. See table inheritance for example (another feature very easily misused but which makes some very difficult things very possible).