Yeah, the way that Plus did it was that certain empires -- the Byzantine Empire, the Mongols, the Aztecs, etc. -- were exempt from the normal empire demotion event. For the Byzantines, it required holding Constantinople -- as soon as they lost it and were below size, they disintegrated to an available kingdom title. As soon as they regained the Queen of Cities, they were promoted back to Empire.
But it makes sense for the ahistorical empires -- they claim their tier by virtue of size and power, so if they lose it, they ought to lose imperial status.