It is not impossible for a crypto currency to fall to zero, bitcoin/ethereum included.
It is certainly possible on paper (51% attacks) - but Proof of Work has so far shown to be pretty stable against this threat.

From a life-savings point of view crypto currencies (majority of them) might be a bad investment. But blockchains as an invention is here to stay, and hence crypto currencies. It is hard to estimate which ones will make it through, which ones will sustain, which ones will receive mass adoption. But my bet is the concept of crypto currencies is here to stay irrespective of ups and downs.