Nothing is guaranteed to last for life. Tether is subject to losing its value in the future because of its utterly-centralized design. A full government crackdown or the issuer "closing up shop", could undermine the value of Tether as we speak. We should be prepared for the worst by cashing out our crypto directly into Fiat instead of a stablecoin. You'll never know when everything will go down the drain as the crypto market is widely unpredictable.
I'd assume that the whole market will crash after Tether goes to $0. That's because Tether is the one stablecoin which moves the market. Ever since the stablecoin launched, the issuer (Bitfinex) has been "printing" new coins with the aim to manipulate the market. Without Tether, the market will be doomed. Prices of major crypto assets will recover after that, albeit at a slow and steady pace. There's no need to worry about Tether since there are a wide number of options to choose from. With a decentralized stablecoin like DAI, there's no need to rely on centralized stablecoins anymore. Time will tell us whenever Tether will survive or become a failed experiment. Just my opinion