if you wanted to back a coin with metal you should be ready to ship the metal worldwide 365 days a year. If i can't get metal for coins delivered to my house it is most likely a scam. (some may not agree but that's how i see it)

You also need a central point for it (the backer).
So to have an asset-backed and at the same time truely decentralized coin is not possible. Or is it?

Unobtanium might be a coin that in the future is backed by metal in a decentralized way. Many investors (each for themselves and privately) could start trading for metal and/or putting out other backing-ideas.
So Unobtanium could be backed with silver by myself and others too in the future.
It's still the future though.