Unlike other elections, you are voting for electors. If Biden drops out or is determined by the DNC to be no longer the real candidate, the electors can vote for someone else. There might be problems with faithless elector laws if he is still alive, but no one has been punished for not voting as directed.
I hear echoes of Nancy Pelosi’s comment about Obamacare — You have to pass the bill to know what’s in the bill.
If Joe has a health emergency, voters can still vote for Democrat Electors — but who will be president? No one will know. They’d have to win the election in order to find out who they voted for.