It is a ponzi/illegal bank scheme the moment they mint more tokens than they have liquid assets.
Could they come clean, will there be a bank/USDT run?
Depends of how much USDT real clients hold and how much USD they hold, and how likely it is that EVERYONE wants to convert USDT back to USD -- it would take a very catastrophic collapse of cryptos for that to happen.
USDT has intrinsic value for money laundering, so even if all other cryptos would go to 0, not everyone would want their USDT converted to cash.
While I don't like USDT, and I don't think that's the case --someone printing USDT could have already amassed a great fortune, and if he was benevolent enough, could probably redeem all USDT of all the real people holding USDT.