Yeah, you see, such an epic collapse would not only be no more likely than an assassination (or natural death, incapacitation due to contraction of leprosy, etc pp) on its own, it would also be rather unlikely to actually lead to Obama bowing out, instead it would simply lead to him losing the general election.
Disagree, collapse resulting in replacement is far more likely than assassination.
One has only happened, to the best of my knowledge, once, before the Civil War (Pierce)--maybe twice if that's your interpretation of what happened to LBJ. Death in office has happened eight times.