Simple; never ever!
If Bitcoins ever crash that severely, they will become worthless. But with tens of millions of dollars being invested in ASICs, which only work for Bitcoins(*), it is hard to imagine such a drop in value.
* Sure, some alt-coins as well, but they are no better than Bitcoin.
They are better in the sense that they didn't award disproportionate wealth to the early adopters.
Bitcoin is like canadian currency in that the land barons who claimed this country started out with huge quantities of the currency by virtue of there being no one competing with them (claiming land as their own vs mining bitcoins before it was a thing) and the activity being almost effortless (saying its my land and I'll kill you if you don't like that vs mining with difficulty of 1).
The result is a Lorenz curve for bitcoin that makes the US look equitable.