Mycroft and John: the Apprentice

Mycroft has been testing and training John to ultimately replace him as Sherlock’s carer from the day that John and Sherlock moved in together (aka the day after they met). 

He tested John to make sure that he wouldn’t sell Sherlock out. Note that Mycroft (presumably) invites Mary to spy on them both and she accepts eagerly, gleefully. I think we can safely assume Mycroft’s stance on that. 
He gave him the signals of a danger night and what to watch out for (”did he accept the cigarette?” “Yes” “Shit!”), and with it, the understanding that Sherlock’s safety must come ahead of any other plans John might have at the moment. 
He makes sure that Sherlock has John’s help in the work that keeps him going (texting him in TGG, sending a helicopter to fetch him from a field in Wales to help Sherlock deal with Irene Adler, etc). 
S: “You phoned him!” J: “Of course I bloody phoned him.” M: “Of course he bloody did.” John and Mycroft both understand that John is to be stepping in here. Remember John’s fury in the lab: “If you were anywhere near this point, you could have called!” He knows he accepted a responsibility here and feels that he failed, and Mycroft knows it. Yet both he and Mycroft also know that John is the one who understands Sherlock better. When John warns Mycroft to go after “Brother mine, don’t appal me when I’m high”, Mycroft doesn’t say a word - he accepts John’s greater insight and just shuts up and goes. 

Mycroft has been one of the most primary intercessors in Sherlock’s life. Evidently he was there for an overdose of large proportion at some earlier point. He was the one who planned the takedown of Moriarty with Sherlock in series 2 and planned Sherlock’s possible escape plans. He was the one who carried out said escape plans and later rescued Sherlock in Serbia. John’s stepped in as the new rescuer, back-up plan, partner-in-crime, the one that Mycroft could never have been. John isn’t afraid of legwork, of shooting the villain, of going after the hound. At the end of TAB, we see Mycroft officially hand over the reins. “Look after him,” he says, despite knowing about the wife that (maybe??) just hacked into the MI5 server on a phone, who shot the brother whose loss would break his heart, who gladly agreed to spy on them. Is this Mycroft’s way of telling John to make a choice here? Either way, Mycroft is done for, and John will need to be there for Sherlock even more than he has been, preferably living with him and keeping him safe and sane and stimulated. And that’s a commitment that isn’t particularly compatible with others, problematic wife aside.