There seems to be somewhat of a misunderstanding here. Gold will not back cryptocurrency, cryptocurrency will (and already is) supporting gold. The cryptocurrency is merely the technology that provides the utility for gold, making it easy to store, track and spend. This is why it is cryptocurrencies with utility that will survive, the others have little purpose other than speculation. Bitcoin as a concept has already demonstrated weaknesses as a technology in terms of proof of work and energy use. Better decentralized ledger technologies are emerging that will make bitcoin and others obsolete.