My pet theory is that when the war is over, the girls try and hold on to life, but as the years pass and their friends and family push forward in life, they slow down. Each day gets a little harder, their steel more stiff and boilers more ornery.
Kongou would be the first, I imagine. Her daughters and granddaughters would be peaceful, the war over, Goto passed on. It would be a short talk to her own grandmother, Mikasa, before Kongou herself would lay herself down to rest on the concrete and stone in the plaza, Goto's ashes resting at the foot of her glass tomb. Mothballed, asleep for decades or centuries, she would rest there waiting for her time to come again. Across the world, ships would go to rest in their hallowed halls, their ferrous reminders dotting the land with their loved ones last remains serving as guards of honor, no matter the form.
When the Abyss returns, so will they. Until then, they will rest the sleep of heroes, sure that their next awakening will be for the entry to their next battle.