If USDT will collapse, BTC and the general cryptomarket will follow. USDT already have the majority of the volume of all the stablecoin available right now and collapsing means most of the crypto user that holds USDT will be in a huge loss which means the buy power in cryptomarket will decrease dramatically.
We know that tether is a really shady company but they are not dumb to runaway with a billion dollar market that they are currently monopolizing. There profit here is consistent and huge.
Bitcoin existed without stablecoins and will exist even if they all disappear in an instant. Of course, stablecoins make it very easy to use cryptocurrencies, however, without them, Bitcoin and other cryptocurrencies will not die. Bitcoin is not directly related to stablecoins, but if something happens to tether, then in the first period this should affect its price, since everything in our world is interconnected. However, I find it difficult to even say in which direction - an increase or decrease, while the price of bitcoin will change.