Bitcoin Being the king of the crypto universe has always influenced the price fluctuations of altcoins and is expected to do the same in the future. Altcoins are used way less than bitcoins and bitcoins have always dominated and are expected to dominate altcoins.
Bitcoin will continue to have the same effect on alts as long as it's the number 01 pair in all trading markets. It's still bigger than USDT and some few stable coins who gained a great position over other alts if we can consider USDT as an altcoin as well.
^ The reason is this, BTC is the main trading pair of all altcoins and before they will purchase altcoins they need to have BTC first.
This will have an effect on the BTC price in the long term, the more altcoins the more people need to purchase BTC as they want altcoins, so altcoins are also contributed to the BTC price that keeps increasing due to the demand.
Does this mean that the price of bitcoin will decrease if one day users stop using it to buy altcoins? I don't think so!
Speaking of USDT, just only diversifying asset but people love to store coin that is highly volatile like BTC.
Exactly! USDT or any other stable coin is mainly used as a trading instrument. Developped in top of exchangers, it shouldn't be used otherwise.