> and for that applications you should build something custom.

Given the spirit of the original post, I think avoiding "build something custom" is the point.

It should be a toggle in the database.

> you don't have a 1:1 mapping between the database schema and the API (and if you do, you shouldn't, since a change of the internal database representation will require a change of the API and will break clients)

I'd argue this is a huge problem. When DB and API get out of sync it creates so much complexity, especially when working in a relational model, and things become impossible to change, or make client caching really difficult.

> The application when updates the data can publish it to whatever subscription system it wants to notify clients.

This is so much complexity though, compared with the experience of Firebase. Good for keeping more devs employed though.