[prmoustache]

There is literally an "overwrite file" command in the file menu.

You didn't lose data because of bad UI but because you are illiterate. You just said it, it warns you. If you can't understand what "none of the images was saved" means, there is no UI that can save you except autosave. But autosave is something you clearly don't want in a photo/image editor, even smartphone apps do not autosave photo edits.

[init2null]

That's a rude and somewhat inaccurate response.

Photoshop has autosave that works well, even for files with hundreds of layers, so it can be done. That being said, I can see that it's less useful when someone chooses not to save.

As for export, a single-layer file should be considered saved when one exports to lossless. A multi-layer file needs a different prompt, and I note Gimp has that now. It flags the file as "Exported to xxxxxx.png" in the Quit dialog.

[prmoustache]

autosave is useful for a file format of working files, like psd if non destructive changes are supported. But it would be stupid for exported end result format like jpeg, png, webp or pdf where changes cannot be recorded.