My guess is there will be quests to upgrade them further. Also note that you already have other ILV90 weapons in the game (same attack power and main stats, diferent other stats like parry or acc) so relics aren't the only best weapons in the game. In addtion to that, they did mention some kind of legendary weapon system where we'll get to see the "best" FF weapons like Excalibur, Gungnir, Aegis shield (yes I know, it's not a weapon) and so on.