Biden is a really unsuitable person for the job, because of his age and health. Why elect someone who will most likely not be in the job for very long.
To your first point, Her Majesty The Queen is 94 and going strong. To your second point, how can you possibly decide when someone will die or become so unwell that they are unable to perform their jobs (to which I might add, illness and disabilities hit people in their 20's all the time preventing them from working)? <- and yes, you did imply he would be so unwell or would die before he completed the 4 years, and truly we are not powerful enough to predict that, nor should we wish ill health on anyone, regardless of our political leanings.
Also, as others have stated, Trump is in the same age group and is surely in worse health with his diet of coke and fried chicken.