Frankly, if they had to make him relapse, they could have used his guilt over what happened to Marcus, or Alfredo. I think it would have worked better, as Sherlock has this habit of blaming himself for everything, and those relationships were much better written, and over a longer period of time.

Oh well. Can't wait to meet Sherlock's father next season, that's for sure. And they've been hinting a lot at Moriarty's return, so there's also that to look forward to!