At the bottom of it all, money is what people believe to be valuable. That goes for Bitcoin as well as for gold or paper money.
The good thing with metals is, they cannot get declared invalid. Paper currency is depending on the backing by a government - which is the quickest way to doom, as history has shown. Crypto currency gets its credibility by being independent from politics plus the fact that it cannot be multiplied. The later, however, is endangered if there are more and more different crypto currencies created, as it happens at the moment. I see a risk in that development.
Precious metals however simply stay what they are, and in the amount that exists. Regardless if it is mined already or not. And nothing can really destroy it. The big problem with PMs is, that the prices for it are completely under control of the ruling powers.