alternatively, they could simply not redeem Tether for USD (which they don't), and then you'd have to hope that any exchanged with USD x USDT trading pairs has enough demand/liquidity to keep the peg.
and of the $2B they're supposed to have, the depth on the kraken order book is barely $6M... and that would drop the price of USDT to $0.30
I have not understood from the beginning why peopled anyway trust them a bit. This is all about trust - if you trust that the money is there - it should be worth 1 USD - but only if you can redeem your USD at least at some point in the future.