Clinton and Trump are the most widely disliked individuals to ever receive major party nods for president. If a major health issue were revealed, allowing for the increased likelihood of a "God called them home too soon"-shortened presidency, could that actually *help* their chances?
Probably not, since Tim Kaine is a first-order garden herb and Mike Pence is a weirdo Jesus squid, so the alternatives aren't any more appealing.
Has a presidential nominee ever chosen a VEEP candidate on the basis of their potential successor's total lack of appeal as a replacement? Do they all do this, because of The Fear?
Just asking questions, here, people.