How low would Bitcoin have to fall before it was no longer recoverable?
Price, in my opinion, would not dictate this. It would have to be something that would destroy the foundation and system on which it is built.
what you say?
As an expert investor myself, I will say that Bitcoin has come to stay and it has a solid foundation that would not be reset to $1 anymore no matter the circumstance of its crash. Regardless of this, assuming it reaches a bottom of $1 per BTC as you indicated, the investors' confidence would nearly be zero and it would be unlikely for it to grow back. If the reason for its crashing is temporary in such a way that the currency could later be vindicated, then it might rise steadily after the vindication is well established to grow back investors' confidence. But if what crashed it is genuine and effective to the point that investors do not want to deal with it again, it might not grow back to a reasonable price in several decades.