When we think of superheroes, and it's easy to include Sherlock Holmes in that category, we tend to think of that starting point called the origin story. It's the place where they truly become who they are. And over the years, many a writer has tried to give Sherlock Holmes an origin: his father killing his unfaithful mother, his sister killing his childhood bestie . . . somehow, like Batman, there must be murder to cause one to investigate murders in the minds of some. But in the Canonical end, our dear Sherlock has no origin story. He simply is.
Watson, however, has a very definite origin story that we're all familiar with. Wounded soldier needing a place to live shares rooms with an enigmatic stranger, who he writes of and becomes a partner to. Watson's origin story is very much suited to the Victorian age of empire if you think about it . . . the British military moves into strange terrain, discovers a unique indigenous people there, then exploits what it found there for Queen and Country.
Too far? Too lefty political? When so many of Watson's adventures with Holmes turn out to be defending or avenging Britain's citizens against Americans, Italians, South Americans, Andaman Islanders, Central Americans, even Asian viruses, and eventually German spies . . . well, it really does make it a fitting sort of origin for Watson.
Sherlock Holmes prides himself on French blood, relaxes in France on hiatus, refuses to follow his brother's path into the British government, and disdains the nobility. And yet Watson is always there to represent the British jury or to have a crush on Queen Victoria (oops, slipped into Holmes and Watson turf there -- but that was SO Watson, wasn't it?)). John H. Watson just grounds Sherlock Holmes as England's man in a way Mycroft never could.
While one might propose that the Stamford introduction is the origin of Holmes and Watson, the pairing we know and love, Sherlock Holmes was going to be Sherlock Holmes even without Watson. True, he might not have survived being Sherlock Holmes as long as he did, but his origin lies elsewhere, always untold. John Watson is the one with the origin, the man with the story.
So does that make Watson the true superhero of the team? Hmmm.