At that point, anyone in possession of the USDT would actually have one big nothing, and that would really be very bad news that would definitely hit Bitcoin. To the average viewer looking at it from the side, it would sound like this: "Coins worth $20 billion printed from nothing, is Bitcoin also fake?"
Dealing with stablecoins is far more dangerous than with fiat -> Most Stablecoins Can Be Frozen, Even in Your Own Wallets
Completely agree with your post. Stablecoins such as USDT goes against the very principle of cryptocurrency. The biggest advantage with Bitcoin is that you will be able to store money on your own. This is not applicable to stablecoins, because the exchanges are storing the money and they are issuing a token to you to represent its value. And as you have pointed out, many of these exchanges reject independent audit and therefore we don't know whether these stablecoins are backed up 1 to 1 or not.
If any of the larger Stablecoins go down, it can have a very bad impact on the exchange rates and adoption of Bitcoin. It is unfair, as Bitcoin has nothing to do with these stablecoins.