I very much disagree that Tether USDT holders can pump bitcoin when the takedown begins. You as a bitcoin holder, why would you sell your bitcoins for something that you know will be worthless after takedown?
In that case the trading volume would decline by a lot. Right now, most of the BTC trading volume is made up of BTC-USDT pair. If that is taken out of the equation, then the volume would decline by more than 50%. The scenario will be different from the one we had after SEC took action against XRP. Back then, most of the users had converted their XRP to BTC, ETH or USDT. But in this case the available options are less in number. I guess a majority of the users will try to convert their USDT to USD or other fiat currency, even if that means paying a hefty markup.