That is what some people are saying. But I don't agree with them as I don't agree with their paranoid arguments. Why should they be afraid of stablecoins when stablecoins are founded on the very price of their own fiat itself? The price of USD falling also means USDT falling. USD dies and USDT also dies.
However, stablecoins such as Libra may really destroy the financial system. But still the fact remains that these coins derive their value from fiat. They share one heart and soul. They kill the system, they kill themselves.
I am baffled about this as well. Maybe they are afraid that there will be lots of highly manipulated pump & dumps from the extremely rich people on the unregulated markets affecting the fiat prices because there are not any safeguards in place like in normal Forex? With enough pressure between Libra/other_asset, the weight of it could affect the other asset.