The thing is, Shatner did come very close to appearing in Star Trek 2009... they wrote a scene involving a hologram of him belonging to Nimoy Spock. Supposedly used as a way of reinforcing to the younger Spock that they would become friends. And had Shatner said yes, you can bet they would've totally have included it in the film's final scenes. It was his reluctance to a cameo and less involvement than Nimoy that quashed it. Not the filmmakers unwillingness to invite comparisons between the original and younger version recasts. The new actors were very carefully thought about when they were chosen, and came with the approval of the originals or their son in a certain case. By enlarge it's the alternative universe lives that have been occasionally criticised (again rationalised onscreen particularly in Kirk's case) and not that they don't bear any likeness. So Khan remains the odd one out, even alongside yet another recast addition, Carol Marcus... whose English accent, again explained... albeit in a deleted scene. So, it's deliberate change in approach for Khan only, in a film continuing to be consistent in other areas.
I still think a blink and you'll miss it glimpse of a very different Khan, some disorientating flashbacks of Marcus finding Botany Bay, some futuristic medics busying themselves around that one unconscious person and Cumberbatch awaking, wouldn't have hurt a thing. It would've meant a lot to those who like such hidden easter eggs. All the while the exposition is coming thick and fast in the brig, with Kirk and Spock trying take it all in... it's just one more thing to hammer his real identity home. Better than... just off to one side... a prolonged, extreme closeup of the actor twisting and contorting his expressions. I'm still not sure whether they applied somekind of morphing effect to his face there, to be honest. Or if that's all the actor's doing. Pretty unsettling either way!
