Mickey Mouse is more widely known in the world than anything associated with Star Trek, and he doesn't look or sound much like "Steamboat Willie." For that matter, the same is true of Superman - also better known than Kirk or Spock.
Poor examples. As cartoon and comic book characters, respectively, Mickey and Superman were subjected to numerous changes in their early years--there was no hardline, established look in the way the right actor immediately shapes a role with traits that are not always born of the screenwriter's pen. The Superman of
Action Comics #1 was already altered by the time AC#6 or 7, where the "S" chest emblem took on a new look, alternated outline colors, along with the color of his cape emblem.
The same can be said of the early, radical changes in other comic & cartoon characters such as Batman or Bugs Bunny, where again, no early, hardline look/character was established--with effect--in the way of a live actor not only in voice, but appearance, as in Garland, Shatner and Nimoy.
To cite Karloff again, nevermind other, non-Universal productions hardly remembered, all one has to do is look at the younger Chaney's take in
Ghost of Frankenstein, or Lugosi's in
Frankenstein Meets The Wolf Man; gone was the established, Karloff
heart of unique performance beneath the make-up which (in no minir way) turned the "monster" into a powerful cultural image starting in the film's decade of origin.
The only result of recasting was how often the public complained about the terrible, lumbering performances, still criticized (or disregarded) today.
If one must use the silly "icon" word, outside a certain local cultural set it's probably the iconography associated with the characters that's recognized, rather than the faces of the actors - Spock's ears, Superman's costume and "S" shield, the Enterprise (which is essentially identical in Abrams's films to every other version).
Spock's ears or the Enterprise do not replace the recognition of Shatner as Kirk, or the whole of Nimoy as the character, whether together or in solo images.
It is the actor's lightning-in-a-bottle shaping of a character which makes a cultural icon. They are not pick n'n swap parts like that found in so many reimagining/reboots to follow. The very reason the nuTrek actors--with all the money and marketing in the world--are not in the same position, two movies in.
he took it away on a holiday to climb the Himalayas where he would be camping and staying in huts without running water and electricity. 
