This would solve a lot of issues related to "where to save data". Using the "OutSystems DB" or an external one.
When the choice goes to an external DB we, developers, need to manage all the changes in the data model "by hand", alter table scripts in dev, then promote this change to upper environments...
In the end, I miss the option of saving data to an external DB but without loosing the platform feature that manages the data model tables, columns, etc.
I know this would raise "joining issues", like not being possible to join that data with User system entity for example but it's a decision the project would need to make :)