Not necessarily. For example, Franz Joseph, Emperor of Austria already had the "I" during his lifetime. Seems he thought he'd have more than one successor. Well, history proved him wrong.



Although she doesn't seem to be afflicted by aging, so it is indeed very unlikely to have a "Celestia II" at any point.

But I think this "I" symbolises more the fact that she founded Equestria, and not some numeration of regents.

Also, it's much more impressive and fun that way, i think.