>If you try to use functionality that the target format doesn't support (e.g. layers on PNG) then GIMP should display a warning dialog about it, perhaps with an option to save as a XCF instead (but not default on it).

That's way more annoying than the current GIMP implementation. I sometimes export the same image a lot of times (to work on reducing gif file size etc.) It's very convenient that I can have a master XCF copy and one-click export without any additional dialogs.

Also JPG is a lossy format, so it doesn't support even saving a one-layer file correctly. Do you really want a supposed "photo editor" to warn people every time they export to JPG?