Luxembourg is “the world’s only remaining grand duchy.” Yeah, so what? Factually, what really is the difference if a country’s constitutional monarch is a king, prince, emperor, or grand duke? What would happen if tomorrow, the Grand Duke of Luxembourg said, “Call me the Emperor of Luxembourg!”? Would the other royal families of Europe stop calling? Would it create a crisis at the UN?
Similarly, could the King of Norway say, “I am no longer king, I am the Great Kloobo and Norway is not a kingdom it is now a Klooboring!”? And what effect would that have? (Assuming that it got past whatever constitutional amendment process was necessary.)
That answers it in the case of Monaco, not for Luxembourg–in fact it begs the question why is Luxembourg a Grand Duchy and not a Principality? Additionally, in Cecil’s column it states that Monaco is a principality because it has always been subordinate to France. What country is Luxembourg subordinate to, now that the Holy Roman Empire is gone?
And lastly, what prevents countries from creating a new royal title in this day and age?
Nothing. Luxembourg is a constitutional monarchy, and the constitution styles the nation as the “Grand Duchy of Luxembourg” and the sovereign as Grand Duke. The constitution can only be changed by Parliament, not by the Grand Duke.
Should Parliament see fit to change the royal title, I see no reason why the rest of the world wouldn’t get on board.