what's so devastating about having Young Sherlock focus on Sherlock before he's a consulting detective and James before he's, well, an evil genius out to destroy Sherlock even if it will kill them both, is that James and Sherlock meet at their lowest emotional and professional points in the series. they see each other at their most abandoned and vulnerable, but it's actually Sherlock who is the most vulnerably open, because James very rarely gives anything away about himself or his history.
Sherlock, meanwhile, brings James everywhere and invites him to the most inner circle of his life. he even trusts him with his own mother. it makes the pit of my stomach churn watching it, because you can just see James filing away all of this information in the way he files away everything he learns about everyone -- it's how James has survived, by remembering everything about a person's situation and what their weaknesses are, because he's a person who has had to work upon weaknesses to get what he's owed.
at the same time, James is so devoted to Sherlock that we know his efforts to help him all the time is genuine. and yet...he's going to remember all of this, and he's going to use all of this one day, too.
And James will be the reason why Sherlock won’t be as open in the future. John will enter the picture and he won’t get the Sherlock that James got.
