[I don't think you're getting this,] Alice says dryly.

People, as individuals, aren't replaceable. Everyone is exactly themselves. The loss of a person is a loss. But except in a very few cases, it's not a loss that Alice personally cares about, and even for those—even for the one he knows would completely take him apart—he doesn't think they are irreplaceable in the sense that the world becomes a permanently worse place every time someone leaves it. It just keeps on being the world, with these people instead of those ones.