Well, that's one hell of an undersell of what a database like PostgreSQL, MSSQL or MongoDB does.

It's not just that people "can't write priginal applications" but that in fact people shouldn't always write their own bespoke single-purpose databases for each application. Getting ACID, MVCC, efficient storage, indexing and backups etc. at the same time is hard, really damn hard. You might get over some of them, e.g., efficient storage, with hardware but there's no free lunch on those topics.

A database is like using a library: You can always write it from scratch (and sometimes you should even) but in 99% percent of cases you should rely on the tried, battle tested existing solution.

> Databases are a means to query and index data with greater performance than without.

I don't think you understand your own point. The querying, indexing, and performance bit are tied to the data structures used internally by the database, not the technology used to persist data.