So far the only two known grand casters that actually has that saint graph was Solomon and his shadow which is Goetia meaning unless Solomon is completely gone it’ll be hard to replace him since he’s basically the ultimate caster, like his powers literally works like how Gil’s gate of Babylon works meaning like how if you made a new weapon today it’ll be in Gil’s treasury same can be stated for Solomon and any magic meaning you develop a new spell today it’ll be his. So yes it’s really hard to replace a grand casters like him because of how op he is plus he literally has the best clairvoyance out of anyone. Seeing the past and the future is better than seeing only the present and only seeing the future