GIMP, like Photoshop, is a complex tool for people who know what they are doing, nobody in that category would be confused by such a situation especially if GIMP shows a warning dialog in case there would be data loss when saving (something that GIMP knows about).
Photoshop has that workflow to entrench its PSD file format. GIMP seems to want to do the same thing instead of treating all formats as equal (technical differences - which can be warned against - aside).
That seems needlessly dangerous in the workflows typical for graphical design. Usually you import one or more source images, make your changes, and export a new image, leaving the source intact (specifically so you still have an original copy of the source images for later use in new, entirely-separate projects).
Sure, this makes it a little bit inconvenient for one-off edits, but I suspect one-off editing ain't the target use case for GIMP, much like how it ain't the target use case for Photoshop.