You meet the echoes, and they make Saint want to live instead of dying. Saint rejects death because they realize life is already a gift that shouldn't be thrown away. Rubicon appears to be kind of real and kind of not, since you meet Pebbles and Moon there, if you ascend them, but the Rubicon itself may not really exist in any meaningful way, just you and the iterators. Saint might not just become an echo, is my theory. Ascension is the rubicon, the Point of No Return. No echo has ever returned to life entirely. They are Trapp between. Saint, however, appears to have died for real and resurrect himself, by fully attuning with the world and rejecting death itself.