I would generally consider dying in office a minimal issue. The VP from the same party takes over, things are fine. The problem is when they decline rather than die and are actually a liability. Biden does seem to be a little scattered in a way that doesn't comfort me. Presidents are heads of state and their words matter. We need someone who can communicate more clearly than Biden (and with less hate than Trump).
Bernie's age doesn't concern me as much rather his general Bernieness. I detest the way he positions himself as outside the Democratic Party with the implication that the Party is bad. I find that largely unforgivable (though because unlike him I am a member of the Party, I would vote for him if he was the candidate). I also think he is fundamentally naive about public policy. It's not just a question of none of his policies could pass...even if he could enact them by fiat, they would be bad policies with all kinds of unintended consequences.
So Biden and Bernie make me grate my teeth for very different reasons.