Backed by something also means the currency is centralized by the backer's ability to honor commitment to redeem the currency for an underlying asset. As far as I know no decentralized mechanism of redemption exists, please someone correct me if I am wrong. What can give Bitcoin and other crypto currencies value is demand for use cases. The more use cases - the higher the demand and the price.
It's probably possible to set a decentralised 'backing' if it relates to other intangible/digital asset.
Yes, backing bitcoin with physical goods would require trust (to the 'backer') but that won't necessarily make bitcoin centralised. Anyone can really back bitcoin with anything. For example, I can publically promise/oblige myself (making formal signed off statement, with all my details etc) to exchange 1 paperclip for 1 bitcoin. Each full bitcoin would be officially backed with one paperclip (as long as I'm alive), but that won't make it centralised. It won't make any difference whatsoever, other than no one could say that "bitcoin has no intrinsic value" anymore.