[There's another long pause as Sherlock weighs his words carefully.]

If there is any way I can help you with anything, John, please...

Rosie is fine. She won't even know you're gone. We will return, John, and everything will go back as it was, and you won't even know that you couldn't visit her when you wanted to.

[There was no way Sherlock could guarantee that, or guarantee either of them would make it back. But they'd done it before, and he was going to be ridiculous and illogical and hope against hope.]