If too many people decide to withdraw their money from any financial system (e.g. banks, brokers, exchanges, etc.), you can expect there to be liquidity problems. These are not Ponzis.

This distinction doesn't even make sense though. The Tethers aren't redeemable directly from the Tether company. People redeem Tether for USD by market transaction. As these Tethers are just USD IOUs, it doesn't seem to matter much whether they are fully backed by USD, as long as there is liquidity to buy and sell them for their full USD value as required.