[It's not that she doesn't understand his side of the argument, here; if she thought saving her world was possible, she would want to try. But she isn't sure that's the case, or that Meridian even knows how. She isn't sure that Zenith can bring about a new world either, though. There's still too many unknowns.]

Of course I miss my world, but how can you be sure that it can even be brought back? You're hurting the people you can see for the potential of something else.

[And this really feels like the most pertinent issue. The people of Zenith are here. The people of Meridian are here. Those are the only things that aren't unknowns. And so that's where Grace's present concern lies.]