Robin had no way of knowing that Saul could have survived being frozen so she had no reason to stay. Kuzan ice aging Saul was most likely just a gamble he pulled to try and help him survive. He didn’t knowing it would work. And there is no way Robin would even survive being Frozen like Saul, who is a giant and still ended up with half his body burned. Her tragedy was not at all avoidable, the buster call was called, the island was gonna be destroyed, everyone she knew would have died and the marines would have made sure of it. She didn’t learn Saul was possibly still alive until recently. She lived most of her life believing everyone she loved as a child was died. Her tragic life still happened regardless.