If people want to pull dollars out of bitfinex, and can't redeem tethers, then the price of bitcoin will skyrocket on bitfinex due to tether hyperinflation (we're probably already seeing that, and have been for months). People will be buying bitcoin at basically any price on usdt denominated exchanges, and start moving them to other exchanges to sell. At that point, you'll start to see price differences between usdt and usd exchanges, as the bitcoin price skyrockets up on bitfinex and drops on gdax.

Probably at some point, bitfinex will become insolvent and shutdown withdrawals entirely. Not sure what will happen then-- probably the price of bitcoin collapses in a general panic.