You can agree or disgree with it, but the arguement agaisnt Catherine De Medicis is clear. It is not that she was a woman, but that was really not that powerful in the grand scheme of things! She was the ruler of France in a Europe dominated by Spain. France alone is not significant enough to be included in the vanilla game when it is in a region of the world where there are already several other nations in the game. It is through its imperial might that France can be justified as a nation to be included in the vanilla game, so why choose a leader without one?
Obviously you will be aware of this, but I feel that many who are in favor of Catherine De Medicis ruling the French in Civ VI aren't quite aware of how much more important a player in world politics France has been at other points in its history.
Under Catherine, only territory was held in modern France, whilst Charles V dominated Europe and the Americas:
Louis XIV, however, ruled a large empire with large amounts of territory in Africa, India and the Americas, in a time when Spain was ruled by the mentally and physically disabled Charles II:
Napoleon dominated Europe probably more so than any other European leader at the peak of his empire, with Spain being among France's satellite states:
And France had one of the largest empires in the world between the world wars under leaders such as Clemenceau, with it maintaining control of its empire through this time, whilst the British empire started to lose its grasp, with the large, resource rich nations of Canada and Australia gaining independence: