One concern that I have for USDT is if the SEC would classify this stablecoin as a security. I know that they are possibly working on that case, but let’s just hope that SEC would not target USDT next. I know that XRP has gone down the wire because of their pending lawsuit from the US SEC, but USDT’s collapse would be total chaos for Bitcoin, Ethereum and other cryptocurrencies.
I don't think that USDT can be defined as a security. It is the digital equivalent of US Dollar, or it is what the Tether foundation claims it is. XRP was always in a difficult position, because they had a 100% premine and more than half of all the tokens were being held by the Ripple Foundation. In short, XRP was like low hanging fruit and made an easy target for the SEC. I don't think that any of the other cryptocurrencies (including USDT) are in immediate danger.