Kirk is Shatner, just as well as Spock is Nimoy, Indiana Jones and Han Solo are Harrison Ford, Dirty Harry is Clint Eastwood, Frank Drebin is Leslie Nielsen... replace the actors and you destroy the characters. It's simple.
Film and television is a medium of vision and sound, hence a character created for that medium is also defined by his appearance and his voice.
Don't try to argue with Batman or James Bond. Those have originally been created on paper only. Batman's face is a drawing with no resemblence to anybody in real life. And Bond has no face at all. But Kirk, Indiana Jones, et al, those have been created on the big TV/silver screen, their faces, their gestures, their way of speaking, etc... are tied to the actors.
Film and television is a medium of vision and sound, hence a character created for that medium is also defined by his appearance and his voice.
Don't try to argue with Batman or James Bond. Those have originally been created on paper only. Batman's face is a drawing with no resemblence to anybody in real life. And Bond has no face at all. But Kirk, Indiana Jones, et al, those have been created on the big TV/silver screen, their faces, their gestures, their way of speaking, etc... are tied to the actors.