Unless the Treasury offered a premium, Treasury bonds would remain outstanding for thirty years. Their duration would decline, and they might have scarcity value for certain, risk-averse asset owners. Many countries do not have long-term debt outstanding. The U.S. government had very little long-term debt as recently as the 1950s and in the 1920s. Markets functioned, so we have reason to believe they would do so in the hypothetical future.
Market participants can solve the benchmark problem by offering synthetic securities.
A bank could agree to accept today the libor rate for 10 years on a fully collateralized contract. This is a marketable, ten year, low risk yield. It could buy or sell 10 year Euro-bonds protected against exchange risk. In these, and other ways, the market can produce synthetic securities that come close to the benchmarks that would disappear. Let me close as I began.