Leto stayed where he was, hand on the railing now, and watched the Doctor discover what everyone else had already learned for themselves, here: there was no leaving. For Leto, it was a reprieve -- for him, he could live here free of the necessity of the Golden Path, free of the guilt he'd feel if he were free to go wherever he chose. And, were he free, he would have chosen to return to Arrakis, to finish what he knew he must. But that was out of his reach, at least for now. He was at the whim of an entity he could not change.
There was a strange sort of freedom in that.
As of yet, no one else had shared his perspective on their circumstances here. He thought of Wash, the despair under his laughter... He watched the Doctor and wondered how much more was going on under the surface for him, too.
"Perhaps you're needed here," Leto offered. It was probably true.