What exactly is the relationship between the magic classes?

[Meliae]

Guess what we became? I am thinking along the main story here. Since even the person who made you one still does not know the reason why they vanished.

actually he knows why they became that, he tells you about it when he gives you final af peice, also as to why blm vanish is simply overuse makes them part of aeyther, like the spirits from ffx where they can still have physical presence but are not physically there, so once they give in or feel content with their work done the void claims them.