If a rematch between those two actually happen, then I am afraid it won't be an election to choose the best person for the charge of the presidency of the United States, but rather to choose the lesser evil.
This happens in many elections.
I personally think that Biden's health won't have any influence on the results of the election, mostly because it seems that there is no serious democrat who is willing to challenge Biden in the primaries and even if there was, none of them would dare to talk about Biden's health, for democrats that is rather a taboo.
I think it's a bit pathetic. Surely there are no better candidates in the US than those two? It just goes to show that it's not necessarily the best who get to run for president, but the ones with the most influence.
Biden has shown signs of cognitive problems and as time goes on he is not going to get any better. Trump at the moment does not seem to have cognitive problems but he will be 78 years old by the time he is elected president again. Ronald Reagan made a joke about his opponent's youth when his opponent questioned his age, because at the time he was the oldest president in US history and these two are older than he is now.