Throwing a failed currency out of a currency basket is like the proverbial tree falling in the proverbial forest. It would not change the value of the Libra at all. The value would already be gone as the value of each stablecoin is computed roughly by taking the entire value of the currency reserves, and dividing it by the number of coins. A good analogy is what happens to the value of a mutual fund when an underlying security becomes valueless.