From a technical point of view I can certainly approve the change. Back in my time I've had couple of my images ruined by badly timed save (user error [tm]), so the non-destructive approach certainly has its merits. However, the new behaviour breaks a lot of ingrained casual use flows. So maybe the best approach would for Gimp to remember the original filename when it's opened and if the filetype is anything except XCF, simply do two saves: create a new $filename.xcf automatically with all the changes the user has made, and also auto-export with the old name.
That way the more casual users could still just do "open, edit, save" for their quick edits. As a bonus, since the XCF file is non-destructive, the original file would still be in there. This might give the best of both worlds: it wouldn't break existing workflows, and it would automatically prevent accidentally changing the original image. Plus it would make the save/export split less of a headache for new users.
Just an idea.