As the old saying goes; just because you can do something doesn't mean you should do it.
> I think that the "use Postgres for everything" messaging was a necessity, even if it is overstated. Use it until you can demonstrate it doesn't meet your near term needs. When that happens, shift.
The problem with this approach is, once "use PostgreSQL for everything" is identified as being no longer be feasible, it has already become an inextricable component underpinning system functionality. Thus making "[w]hen that happens, shift" extremely difficult.
Contrast the above with only using PostgreSQL (or any other RDBMS) to manage data and their relationships, eschewing stored procedures as well, and the "when <insert condition> happens, shift" decision becomes much more feasible to entertain.