What one factor could you throw into any universe where Sherlock Holmes exists and completely destroy the Canon?
Is it some different sort of Moriarty? Giving Holmes brain damage early on? Making the rents on Baker Street too high?
Nope. The one thing that would kill the Canon more than any other is simply giving John H. Watson a television.
"The reader may set me down as a hopeless busybody," Watson writes in A Study in Scarlet, "when I confess how much this man stimulated my curiosity, and how often I endeavored to break through the reticence which he showed on all that concerned himself. Before pronouncing judgement, however, be it remembered, how objectless was my life, and how little there was to engage my attention."
Okay, I guess I'm talking about hetero Watson here. Gay Watson doesn't need an excuse to fall in love with his room-mate. Hetero Watson, however, is only paying so much attention to Sherlock Holmes because he doesn't have a television, or an internet connection with YouTube and Netflix.
It's hard for us, at this point, to imagine the Victorian condition of lying about one's apartment with absolutely nothing to distract one's self, except wondering what the room-mate is doing. Had Watson been binging on Gray's Anatomy or The Bachelor, the comings and goings of his room-mate would not have attracted his attention nearly as much, and the poor health which limited him from venturing out in A Study in Scarlet would have handed him over to demon television in an instant.
It's interesting that even our modern Watsons on BBC and CBS don't spend any time indulging in the medium they owe their existence to. Both are healthier than Victorian Watson, yet neither seems to bother with Big Bang Theory, a sitcom that would definitely have them writing their weird new room-mate off as "a crime nerd" and ignoring his unusual free-lance job. (Unless they were in America, in which case, Watson might be mainly pestering Holmes on how he was going to get health insurance with random consulting fees as an income.)
Given his medical issues, Watson still made it back from the war without any pain-killer addictions or obvious alcoholism, so maybe he was the sort of person whom television would never have been a temptation. But if he had gotten hooked on "his shows" upon his return from Afghanistan? The Sherlockian Canon as we know it would never have come to be.
And that probably says something about where so many of us are today, he wrote as he went back to finish watching The Cloverfield Paradox on Netflix. (Well played, Netflix. Well played.)