What is the story of Roland's final turn?

In the March 2, 2022 episode of "The Kingcast" podcast, King affirms that Roland's final turn is the one where he doesn't let go of Jake under the mountain. No, it's not the one where Roland has the Horn of Eld. Possession of the Horn is a difference, but it's not the marker of the final turn. Given that, what is the story of Roland's final trip through Midworld? If he doesn't drop Jake, there is no need to retrieve Jake at the Speaking Ring. Susannah doesn't get pregnant. There's no need for Susannah, Jake, and Father Callahan to go to New York City. There's no Mordred. What's the last story of Roland?