I've been pondering this and thinking that although they are very much a duo, Sherlock is still the central character, and everything relates back to him. For instance, although Mary was John's wife, her story affected him personally, partly through John and partly through his own relationship with her (and how much he cared for her and his vow to her). Obviously losing Mary was John's story, but it was also very much a story of Sherlock's loss (of Mary and John) and reconciliation too. In contrast, for instance, Sherlock's relationship with Irene was key to ASIB, but John didn't really have much relationship with her at all. The same with Moriarty, to some extent. Eurus, obviously. Even characters like Jeff Hope and Culverton Smith, I suppose - they have a particular relationship with Sherlock and not so much a relationship with John.
Sorry, this is kind of OT, but the whole "how much backstory do we want" idea got me thinking about it, and I suppose I hadn't noticed before how much the show is Sherlock-centric. Of course, it's called "Sherlock", and Sherlock is the detective! But I hadn't really noticed in terms of relationships - the relationships of the clients and the villains to Sherlock are important, but their relationship to John is secondary. Even (to get back OT) with Mary whose major relationship by far is with John, the writers created a relationship between her and Sherlock, with him going as far as vowing to protect her (when they could have still have had a "Mary" case without that relationship).