In the case of McCain, you can at least hope that his successor would be a good choice.
No reason not to think McCain’s VP will have most of his positional problems. And really given the ages and histories of the two Obama is a lot more likely to be alive 8 years from now than McCain. Depending on his genetics the smoking might not seriously effect his life expectancy, he could always pull a Yul Brenner or Paul Newman (both of whom got well past normal life expectancy before developing cancer). McCain’s already been playing tag with cancer for at least 8 years.
Of course if you really want to take the grim view we’ve all got one foot in the grave, there’s only one way out. And all presidents are more at risk than the general population.