wait, sorry i just thought that magicians can still lose their abilities, since what was given to them in the first place can still be taken, so their abilities aren't a permanent fixture but a gift to them. and meredith did say to rood that the giant or himself if he's indeed the brother gave them that power to fight off monster because the human king begged for help. so if were going by the theory that meredith is the brother than everyone is still drawing their source from meredith. so it didn't matter that the giant gone because meredith is still alive and was the one who actually saved them that mana to use in the first place right?? meredith did say the giant/ himself was the one to give them the mana.