And if you're going to split hairs by saying DHL, USPS, and FedEx are considered third parties, then so are Comcast, Verizon, and other ISPs who provide you the means to transact.
Yeah but that third party is everywhere, available to all (US isn't the only country in the world you know?) and isn't aware you are using his service for sharing cryptocurrencies or simply reading Hacker News.
The current third parties for transferring money, well they all fail one or multiple of theses while cryptocurrencies dosen't.
Stablecoins, on the other hand, have zero advantages over U.S. dollars. Backer goes bust, someone steals the dollars, someone freezes their accounts, et cetera and your currency is worthless. A trusted third party rests at the centre who is less regulated and guaranteed than a bank.
(I'm sceptical that these schemes will pass AML muster. Using a stablecoin over U.S. dollars makes sense if you're (a) incompetent or (b) laundering money.)
The real issue here is getting a USD-ETH oracle.
Cash.
(There is no way to send electronic dollars without involving a third party, though more money is laundered using dollars and euros than cryptocurrencies. I'd argue leaving a permanent, public record of transactions is a poor price to pay for decentralized electronic transmission, particularly given the centralized issuer and guarantor problem.)