A Postgres database, for example, with correctly-limited user permissions set, and a good set of views and triggers and stored procedures, is exactly equivalent in power and flexibility to putting some blobs of Sinatra API-routing and model code in front of Postgres-used-as-a-dumb-object-store.

RDBMSes were always meant to do, on their own, exactly what microservices are doing now. That's why they're so much more complicated than just being dumb object stores: you can encode within a RDBMS schema the constraints of the model, and the valid transformations you can make to any given piece of data, to create an API exactly as narrow and high-level as a given user needs.

I think it was just some quirk of their use in mainframe contexts which lead to their auth protocols never being hardened to work on the public Internet, which meant people started to believe there was something inherently insecure about letting users talk to an RDBMS.

There's not; a database, a real database, is a persistent micoservice server.

And now we're all the way around to reinventing them on top of themselves, but this time they're three separate process-spaces, random custom languages are required, and they don't understand questions phrased as relational algebra.