Sanders will turn 79 a month after election day in 2020. By the time his first term would end in January, 2025, he'll be 83 years old. If he's a two-termer, he'll exit office just after his 87th birthday.
The human body really starts to go to pieces after a certain point, and a president who will be in his 80s most of his term is really asking for America to have to get very familiar with a lot of end-of-life illnesses.
Nobody cares...I'd rather have Bernie croak in office than trust a DLC stiff. If anything...Bernie dying might help in getting legislation passed...much like LBJ exploited JFK's death in ramming though a ton of legislation.
Him dead in office isn't the worst case scenario. Him going through a lengthy, debilitating, illness that prevents him from handling the duties of state is.
Yes, the 25th Amendment exists, but any attempt to use it will be denounced as a coup, especially if Sanders isn't himself ready to relinquish the reins.