Sherlock Holmes was a literary character open to interpretation and adaption. Khan was as much a visual and performance based characterisation as he was written one.
A spurious distinction. Fictional characters are fictional characters. There was a time when only one actor, William Gillette, had ever played Sherlock Holmes -- in the 1899 play that Gillette cowrote with Conan Doyle. Gillette's performance was iconic and introduced the phrase "Elementary, my dear Watson" and the calabash pipe to the popular perception of Holmes. I'm sure that back then, there were people who were upset when H.A. Saintsbury took over the role for the touring company, or when John Barrymore or Basil Rathbone took over onscreen. All these things are relative. For all we know, a century from now, a dozen different actors may have played versions of Khan. Just because we've only seen two so far, that doesn't mean it's intrinsically wrong to recast the role.
I mean, come on, Montalban only played the character twice. I can somewhat understand the attitudes of people who resisted the idea of anyone other than Shatner playing Kirk, because he had over a hundred opportunities (counting the animated series) to delineate and solidify his characterization. But Montalban didn't even play the same version of Khan in TWOK that he did in "Space Seed." The two had the same name, but one was a cunning manipulator and seducer with an aim toward galactic conquest and the other was a raving madman out for personal revenge. So I'd hardly call that an indelible, definitive characterization.
TWOK has not been surpassed by STIF in terms of acclaim and the fact is that Cumberbatch was very much having to step into a role where he would automatically be compared to an iconic performance. That is different to an interpretation of Holmes.
I don't see how. There have been many iconic performances of Holmes. People of my generation grew up seeing Jeremy Brett as the absolutely definitive Holmes, and before that many saw Basil Rathbone the same way. You may think your objections are unique to this character, but I've heard the same kneejerk resistance to novelty from many Holmes fans in the past, people who refused to accept anyone other than Brett in the role. I've seen fans complaining about Robert Downey, Jr.'s Holmes in much the same tone that you're complaining about Cumberbatch's Khan. You may see a difference, but I don't. I've heard it all before, about many different characters.
He was always destined to have a hard time breaking away from that and the attempts to do so have reduced him Khan to something of a largely unfamiliar bad guy who just happens to share the name. I wouldn't be the first person to suggest that had he remained John Harrison it would've made little difference to STID. Therefore I think having him carry the burden of the Khan name was a disservice to Benedict. He should've been able to create his own bad guy from scratch (in terms of performance).
I'm sorry, but it sounds like you're saying that Khan is a more iconic and familiar character than Sherlock Holmes, and while that may be true for a few very insular Trekkies, in general it's quite a ridiculous notion. Khan has only been depicted
twice before, not counting tie-ins. And, again, in two very different ways that I don't even consider the same character. Holmes is an immensely more well-defined fictional persona. There's thousands of times more cultural baggage attached to Holmes than there is to Khan. Khan is a pop-culture lightweight, inconsequential in comparison.