It is not an unfair question. While the standard that the vice presidency is “only a heartbeat away from the presidency” has become a cliché, it is also accurate. Four vice presidents have become president through the natural death of a president, four through assassination and one through resignation. That’s quite a number considering we’ve had only 43 presidents.I guess it's all in how you frame the issue. I'm already a little sick of the "only a heartbeat away" line, especially when you consider that the other side of the aisle is trying to put someone who is (I think, anyway) just as inexperienced less than a heartbeat away--in the oval office itself.
I'm not sure you can count Ford, who became president after Nixon's resignation. The "heartbeat" issue--at least when I've heard it used--has always been in regards to the possibility of the death of a sitting president (generally a jab at McCain's age). Of the other eight, four have been "promoted" by the natural death of a president:
John Tyler (William Henry Harrison)
Millard Fillmore (Zachary Taylor)
Calvin Coolidge (Warren G. Harding)
Harry Truman (FDR)
And four by "unnatural" deaths:
Andrew Johnson (Abraham Lincoln)
Chester A. Arthur (James Garfield)
Teddy Roosevelt (William McKinley)
Lyndon Johnson (JFK)
You'll notice that only three of those happened in the last hundred years. I'm not sure that's "quite a number." We've had more wars in that time period. People living today may have seen it happen once...maybe twice. (I've never seen it happen--again, I'm not counting Ford.) Even the "natural" deaths are exceptional circumstances. Harrison, serving in his late sixties, died of pneumonia and its complications before medical treatment had advanced enough to even be aware of microorganisms. Taylor also died in his sixties (65) of a mysterious illness. Harding died at 57 from either heart attack or stroke while also apparently suffering from pneumonia as well. And FDR, also in his sixties, suffered from (most likely) Guillain-Barre Syndrome, which can be fatal. I'm not a doctor, so I don't know the likelihood that it may have caused the cerebral hemorrhage he died from. The point being that advances in medicine have extended life expectancy and the effectiveness of the Secret Service has reduced the chances of assassination. Three in the last hundred years isn't that frequent, and with the aforementioned advances, I would argue it's likely to be even less frequent in the future.
Look at it this way--you may be able to name the four presidents who were assassinated. Did you know the VPs who succeeded them? Could you have named the first four, who followed the natural deaths? Do you think most people could? Try this--ask friends and family to name more than four vice presidents from, say, 1850-1950. That's a hundred years. If Simon is right, and the possibility of a vice president taking over the presidency is so common, five shouldn't be a problem, right?