Yeah, there's no left credible left challenge to Clinton because there's no figure with the stature to become a voice for that challenge.
It all depends on the national mood. It's hard to say where we will be in a year and a half, but putting boots on the ground in Iraq is a real possible. If the conditions are right, even Bernie Sanders could mount a serious challenge from the left.
Ignoring all of Silver's factual arguments by responding with a vacuous "anything can happen in a year!" or a "a week is an eternity in politics!" is just
the worst. Hillary got nearly half the vote and only barely lost even in a perfect storm against her, and she's in a much stronger position now than she was then.