First of all, which commodity should the value of bitcoins be based on? Gold? Then Bitcoins would just mirror the gold price and we wouldn't need cryptos. The same effect we could have by buying ETCs on gold. I think we are past the time where we other to tell us the worth of things. Just because there is no big central bank behind bitcoins guaranteeing it's valu doesn't mean it will be worthless.
Some people would argue that, in the case where cryptocurrency is to be backed by a physical commodity such as gold or other precious metal, its existence is quite justified since that physical commodity, while being valuable, cannot be transported to another place trustlessly, costlessly and securely. Bitcoin solves the problem of lack of portability. But in my view, this whole concept, I mean pegging cryptocurrency to a commodity, doesn't make much sense, especially from the standpoint of the "finality" of transactions. When you make a regular transaction in bitcoin, you essentially hand over your bitcoin like physical cash because it is, in essence, a bearer instrument where all transactions are immutable and final. When you try to back it by something else (e.g gold), transactions stop being considered final because the underlying asset hasn't even moved. To make a payment, you need to relocate gold physically, but that means we haven't achieved anything, and gold is again subject to centralization and government capture. Bitcoin pegged to a physical commodity is not "real" money, it is a money substitute, questionable claim on something valuable, essentially, it is a form of IOU.