Because it really annoys me that John and Mary, who are not only supposed to be Sherlock's best friends but also medical professionals, didn't notice or care that he was getting weaker and clearly needed medical attention from rushing around London with a hole in his chest all to try to fix their marriage.

So my prompt is; Sherlock didn't call an ambulance when he got to 221b and he just collapses during the conversation and they're not able to save him in time. He dies there in 221b between the both of them.

Major guilt and grief from John and Mary follows.