Stablecoins backed by USD dont matter to US citizen, it would only matter to foreign citizens. There are only are finite amount of USD circulating in the world, the US does print more and more every year in T-Bills, but they are finite until the US buys T-Bills back to pay off debt. If you sit in China lets say, its hard to just walk in a FX exchange and exchange unlimited chinese yuan to USD, its highly regulated and China uses its limited (limited, but large) holdings of USD for other things (like pay for oil). There is an entire convo around FX rate manipuation when pegged to USD. How USD stable coins can circulate is unknown, but if the fritcion is lower, and in a secondary market to developing countries, someone will pay a premium on $1 USD converted to stablecoin.