It seems that in the next election, Biden will again face Trump and this will again be a fairly hot election, considering that these two pairs are people full of controversy. It will be quite difficult to choose who will win, whether Biden or Trump, but considering Biden's declining health, this will be a good chance for Trump to regain his position and remove Biden from his position as president.
I am not sure whether health of either candidates (let's not forget the fact that Trump is 77 years old and this is no longer 2016) will be a deciding factor during the 2024 POTUS elections. A lot has been discussed on this regard, but Democrat voters simply don't mind whether Biden is in good health or not. Most of those who make noise on this issue are Republican voters. There is always a chance that Biden will be replaced by someone like Kamala Harris after a year or two during the second term (in case he gets elected). Getting Harris directly elected during 2024 will be next to impossible, given her radical views and lack of popularity among the moderate voters.