The problem is "Prolog is the database" is that there isn't a great solution for persisting that database, which is usually what people mean when they talk about databases. Sure, for SWI-Prolog there's Persistency but that a) doesn't help you if your data doesn't fit in memory, and b) doesn't give a lot of the same guarantees as something like Postgres about durability through sudden failures (as far as I can tell, the docs sure don't mention it). It's closer to SQLite than a database you'd use for a production web app.
Yes, I agree with this. In SWI-Prolog, in addition to library(persistency), there are already some published results on transaction support for the internal database, which will give you some features that facilitate such use cases. This is already available and in fact also running for production web apps, but currently only for the RDF database. It is true that if you need more advanced functionality that dedicated database systems readily give you, then you have to wait until such features become available in SWI-Prolog, pay for their implementation, or resort to using an external database for which there often are bindings.
