If the crash is bad enough: Tether itself. Or Bitfinex, its parent.

If they buy up their own debt at $0.01 on the dollar then they got away with their fractional reserving scheme, and their executives walk free. Free and very wealthy.

There is always a clearing price. The question is simply "how low"?

My experience in crypto tells me that there is usually always someone willing to be on the other side of the trade. People exiting Tether would still be exiting through a smaller and smaller door of BTC and ETH going up and up. More and more desperate to pay any price to exit Tether.