(* Database: Should be a commodity, but isn't due to vendor stubbornness. *)

You can always write your own or use an open-source one. You don't have to worry about using SQL if you don't like it and just want small code.

(* Difficult to write (I don't mean the SQL, I mean the DB engine). *)

Perhaps if you didn't use SQL it would be smaller.

(* Extremely performance-critical code. *)

OR-mappers are not known to add speed to systems.

(* Monolithic in design. *)

Please explain this word. You keep using it.

(* app framework: Written in a high-level-language, so it's easier to maintain. *)

There are small RDBMS written in Java. I can't think of the name right now.

(* Open source--tweak to your heart's content. *)

There are multiple open source RDBMS. For example, MySQL, PostgreSQL, SQLite. Most are written in C I believe.

(* Any given subsystem is easily wrappable if swapped out. *)

I am not sure what you mean here.