Casting Watson as female must have narrowly edged out having a black Holmes. Just transplanting the story from Victorian-era England to modern-day America was never going to be enough afterall.....
I could certainly have gotten behind, say, Colin Salmon as Holmes. Why not? Holmes and Watson are two of the great roles of English literature, and they've been played by many different actors and interpreted in many different ways. Why should only white men be allowed to play them?
And for that matter, why shouldn't they be transplanted to different eras and cultures? There have been a number of Holmes pastiches set in outer space or the future. Heck, we're just coming to the end of an 8-year run for a TV series that's basically Holmes as a brilliant doctor (namely House
, which is playing off the fact that Doyle based Holmes on a real-life doctor he'd known). The great archetypes are always open to reinvention and variation.