I don't see why Twice has to give it up. Is there a rule somewhere that there can only be one Nation Girl Group?
SNSD can still be the NGG while Twice is too. So now we can add another one in. SNSD, Twice, and the new group. That's good company to be in.
I'm going to be brutal -- sadly the general public moves on quickly so older groups aren't seen as the NEXT HOT THING but you're right that both SNSD and Twice are the NGG for different reasons and will always be.
People will say "X flopped" but that's not true.
You're right that at the end of the day, there's a certain standard that both have achieved.