While many good points have been made here regarding generational changes in actors portraying characters, and who is "definitive" in a role, I would still argue that both William Shatner as Kirk and Christopher Reeve as Superman are a bit different than the norm.
Understand, I'm not talking about which performance you prefer or which episodes/films you prefer. I know very well that there are many people who, for example, prefer the George Reeves television series to the Christopher Reeve films, and that's fine. I know there are people who prefer the Chris Pine portrayal of Kirk to the William Shatner portrayal. Also fine.
What I'm talking about is who the public thinks of when a character is mentioned. To this day, despite the success of subsequent efforts, I would stake large sums of money on the fact that a majority of the public still associates William Shatner with the character of Kirk. And I would venture to say the same is true of Christopher Reeve as Superman.
James Bond, Batman, Sherlock Holmes, Doctor Who... all of those are examples of characters that have been played by many different actors, but the difference is that I don't think one particular portrayal has ever "stuck" in the public consciousness at a significantly higher level than the others. As much as Christian Bale has been lauded for his portrayal of Batman, so was Michael Keaton. And I'm willing to bet a nice sizeable portion of the public still thinks of Adam West when someone mentions Batman.
But for Kirk and Superman, there's something different. Something iconic about those particular portrayals. Something that's "stuck." Not saying they're the best portrayals. Just that they are embedded in the public consciousness in a way many other portrayals are not.
IMHO, of course, worth exactly what you paid for it.