Since it's very difficult to convert USDT into real USD, if people were desperately trying to exit their Tether positions, they'd likely do it by changing for other cryptocurrencies like bitcoin.
I think that would be a spike in Bitcoin relative to Tether, not relative to the dollar. In the extreme, people might want to exchange any amount of Tether for the tiniest piece of a Bitcoin, but that doesn't necessarily mean that the Bitcoin/dollar rate changes.
Why ignore any other negative impacts like, lack of confidence in crypto from USDT collapsing because crypto is still a Wild West having a random corp printing money at will and there will certainly be a group of people who will face huge loss when billions just evaporate and the impact all the exchanges take who are dealing with USDT? SEC is not going to like it and can probably lock ETF for a good while.