So I ran a life expectancy calculator. Both Trump and Biden would have more than a 75% chance of finishing their terms. They project that Trump will live to be 89 and Biden will live to be 95. Biden would have a 25% chance of developing dementia before he left office, while for Trump it would be about 8-10%

Please note that I made some educated guesses about the current state of their health and so these numbers are not precise.

The life expectancy figures were calculated using the following website. https://www.blueprintincome.com/tools/life-expectancy-calculator-how-long-will-i-live/

My main takeaway is that if either Trump or Biden is elected President, the odds are quite good that they would finish their terms.