Bitcoin is backed by energy. Its Proof of Work requirement is essentially a proof of expenditure of energy. You cannot spend a unit of energy more than once. You can spend it on mining Bitcoin, mining another PoW coin, or something else like running your household. Energy is a commodity that has a price, and it cannot be counterfeited.
Now Proof of Stake coins... that's another topic. There's nothing backing those. You can hold an infinite number of different PoS coins. Inflation through diversification into new PoS coins will ultimately destroy purely PoS coins.