So the question is: Why have two copies of your data, two products to learn and monitor and operate, write boilerplate to move data between the DBs, etc.?
A "message queue" comes down to being another index on your table/set of tables ordered by a post-commit sequence number. These are things all SQL DBs have already, it just lacks a bit of exposing/packaging to be as convenient to use as a messaging queue.
So the question is: Why have two copies of your data, two products to learn and monitor and operate, write boilerplate to move data between the DBs, etc.?
A "message queue" comes down to being another index on your table/set of tables ordered by a post-commit sequence number. These are things all SQL DBs have already, it just lacks a bit of exposing/packaging to be as convenient to use as a messaging queue.