Well considering the gokage, had never fought in a team formation together, and they were up against a undying edo beyond prime madara(regardless of whether he was going full out the entire fight), they did pretty good.
Its kinda annoying seeing people bash them, and label them as fodder, when its not that they are weak, but edo madara is just that much stronger than them.
The kages surviving isnt a big deal, or illogical. Its if they dont have any sort of side effect from takeing that much damage(ie lifespans shortened)
Tsunade and oonoki, should definately die.(after the war is over) because oonoki is old already, and tsunade has used her byagou so much, that her lifespan should be approaching its end(as she is a withered old hag , and she is only 50-60 years old.)
The side effect of byakgou(and probably all major healing) is a shortened lifespan. Because a persons cells will only reproduce a set number of times in their life.