I do not think that tether can somehow affect bitcoin, it is not powerful enough for this.
Tether is a powerful token and it does affect the values of bitcoin and other cryptos because investors and traders do also buy and sell stable coins such as tether on a regular basis so that they can secure their profit from the hazards of unstability .
In addition, btc is the largest coin on the market and I doubt that anything at all can greatly affect it.
Not all can affect bitcoin . Your right ,bitcoin is the largest coin and bitcoin is the one that can affect other coins the most .
Bull run will always happen with or without the help of stable coins , that is for sure .