It's one of those pedantic programmer things I think. It's strictly better semantics, but I'd agree it's poor UX.

I wonder if it came from LibreGraphics collaborations as Inkscape did something very similar.

What's weird is that The GIMP used to win plaudits for allowing user-set defaults (particularly it's shortcut system). Why this change, "save only allows native format", wasn't introduced as an option I don't know.

You can "Export" on any format :P

I know you can. But for someone who mostly uses GIMP to crop screenshots, for example, the workflow is used to be: 1. Load PNG 2. Edit PNG 3. Save PNG 4. Quit

Now it's 1. Load PNG 2. Edit PNG 3. Export as PNG (retyping the extension) 4. Quit 5. Say that yes, you do want to quit even though there are unsaved changes (because there are not). 6. Worry about whether there really WERE unsaved changes after all.

I fully understand why graphics professionals prefer the XCF-only approach; what I can't understand is why the GIMP people flatly refused to let the rest of us use the software the way that suits us.

Alt-F + W will save your file back to the original file.

(or File menu + "Overwrite {filename}". I complain a lot about GIMP's UI but I don't really see a problem here.

What if you don't want to overwrite the original?

Then the "Overwrite original" option is probably not for you.