I'm not sure what you are implying or how we got from "why it was originally designed that way" to "still not available".

If you still want talking about notions and time, GEGL as the non-destructive backend for GIMP was started by Rhythm&Hues developers in early 2000.

We could have a long conversation about priorities and availability of developers and refactoring and all the reasons resulting in non-destructive editing not yet available, if you like.