Cassian takes a moment or two to absorb this line of reasoning. "I'm not sure if time travel comes into it. I think it's just very far away."
His expression turns pensive, then; the thought hasn't occurred to him in quite that way before. He's been thinking of himself as dead - all ties severed, all debts cancelled - or else as being still on that last mission, somehow, Jyn's mission. Working with her for some objective he can't define anymore. Either way, still connected to the universe he knows.
He ought, Cassian tells himself bleakly, to know better than to make that kind of assumption.
But he meets the boy's half-smile with one of his own, encouraging. "What were you hoping for, exactly?"