Sherlock is very serious for a few moment. "Please, don't change," he says. "I lost you, Mycroft. You changed. I don't blame you at all. I blame everything else. But if we get home again, please don't change." His other self, wherever he is, could really do with this Mycroft, easy and smiling and young. Always young.
"Last mission," he repeated with a nod. He didn't remember that as it happened in real time, because Mycroft kept things like that from Sherlock so that Sherlock didn't freak out and try and top himself, which wasn't always a possibility, but certainly the thought of losing his brother wouldn't have sat well with him. So Mycroft's missions remained a secret and then, when he came home, Sherlock would guess where he had been. He knows that Mycroft survives, of course, but this isn't going well.
"It's going to be okay," Sherlock says, because Mycroft looks like he needs reassurance and, for once, he wants to be the one that can look out for him. "It's going to be okay, Mycroft. Just, stay here with us. For a while. Get fit again. Try not to catch whatever I've got," Sherlock smiles.