It's halfway what I'm talking about, thanks. Absolutely yes to the idea of being able to query everything!

But for me the idea of writing (CREATE, UPDATE, DELETE) would be an integral part of it as well -- whether inserting a property into a JSON file, adding a cronjob as column parameters rather than a text line, or creating files.

The writing part is what NixOS does (with Nixlang rather than SQL).

See also https://augeas.net/, which presents structured file contents as navigable paths that you update.

Red hatters maintained it (hung out in irc) until cloud container workflows surpassed in-place system administration. It pairs nicely with puppet, and can be used manually.