Imagine if Bitfinex shut down with no warning. All the USDT stored in Bitfinex would simply be lost so it wouldn't cause a bank run. But massive amounts of USDT would flow from Bittrex, Hitbtc, and Poloniex to Kraken where the price would drop to pennies due to lack of liquidity. Maybe then Kraken suspends trading "to calm the market" and never resumes.

I am reminded of the default of BTCST which I guess no one remembers.