Gold does not have to be backed by a commodity, because it is a commodity.
I don't agree that it's useless.
I don't agree that perception is reality. Reality is what is is. regardless of what people believe it is.
So what will happen to gold price when suddenly everyone stop perceiving it as valuable? What if no one buys it as an investment or store of value and the price is solely dictated only by its practical usage (such as electronic and aerospace components)? Will it remain 10% of its current value?
The fact is that most of the gold's price is created by the general agreement that it is precious and valuable, so the 90% (out of the ass estimate) is backed only by perception.