What is the relevance of Rosie in series 4?

 (Sherlock meta by mae-jones)

Q: [...] What do you think is the relevance of Rosie in series 4? Rosie is seen for just a few minutes. Hope I could get an answer. Thanks!

A: [...] Why did they bring a baby into this show? Well, I honestly think Mary becoming pregnant was an effort by the writers to remove all doubt that John and Mary were legitimately in love. It was almost necessary given how skeptical they knew some fans would be. They were setting Mary up to be morally compromised which meant people would eschew her motivations and whether she really loved John. So, if you look at the timing, the writers were very deliberate in these steps 1) John falling in love and making the decision to propose all on his own 2) John and Mary marrying with Sherlock’s full endorsement 3) Sherlock deducing Mary was pregnant at the end of the wedding to head off the idea John felt pressured to get married 4) The revelation that Mary was an ex-assassin after these bonds were all established.

Rosie, imho, was a literary device meant to “prove” John and Mary’s love for one another (in several ways, I could go on). The writers tried to make their falling in love unquestionable. It was actually painfully obvious from a writing perspective how hard they tried to shut down every argument people might have against this couple getting together. Yes, a similar story could have happened if Mary miscarried or something but it wouldn’t have been as definitive and considering they intended to kill Mary off, that would have just been tragedy heaped upon tragedy. As it was, Sherlock became pretty dour in Series 4.