Who will know if the internal format changes? GL can still report that format. All of the appropriate functions can still pretend that this is the internal format. Indeed, the internal format is more of a hint than forcing of any real behavior. You can emulate any of the internal formats with RGBA8 (except for those with higher precision).
I can live with a driver doing some copying, too. As long as everyone knows that it’s possible, people will stick to formats closer to what the framebuffer offers.