The volume is tether is very high because most peoples trades their cypto coins against USDT.
btc/usdt, eth/usdt, xrp/usdt, etc. that's why the volume of tether is very high, but it is a quite wrong to call it 'world most preferred coin' by the trading volume in coinmarketcap.
It is not going to affect Bitcoin. People don't want to pay for things that was built in thin air.