People let politics influence their views on these two. It is pretty clear Biden is not as sharp as he was previously, however, he is also a 78 year old man who has spent nearly his entire life in politics. It is possible that he has some form of early on-set dementia or other mental related disorder, in fact it is likely given his behavior. That being said, it is pure speculation and people can live many years with such issues as dementia or Alzheimer's as this site shows pretty easily. Trump is an obese, former (or current) drug-user who doesn't look particularly well most of the time. Fairly obvious that 1. Biden should survive not only his term but likely years in the future (death does come for us all eventually though). 2. Trump living another decade is less likely than Biden living another decade