It's a bit odd.
Since people cannot cash out USDT to USD, every USDT that is bought with USD represents $1 that will never leave the crypto market.
So when buying USDT with USD, no real value is added to the cryptomarket, and yet when USDT would "cease to exist" 2.5 billion USD will be gone from the crypto market. How? Because of the deception in how value is being defined and calculated. If something cannot be traded for $1, then why would it be worth $1?
Is it really the case that USDT cannot be converted to USD? In that case it is established as a scam, regardless of what the other facts may be and whether they are all "backed up". From their website, it was initially described on how to convert USDT to USD. Then there were some issues with their banking relationships in certain countries, and customers from those countries were told that they would not be able to exchange USDT to USD for the time being. But my understanding was that it was still possible for persons in certain countries.