now Bitcoin price has increased 17.5% in the last 7 days and now the price is $27,750.69, then BTC Domination is 70.9% with Market Cap amount $727,677,130,354, but Ethereum price is currently $644.61 and is down 1.44% from the last 7 days, so I bought Ethereum with all my money (money that I use specifically for investment), because I believe Ethereum price will soon increase very high (bull run) following Bitcoin price movement and of course I will get a very big profit from investing in Ethereum (my target for Ethereum price is $1100-$1400), because I believe if Bitcoin dominance decreases, then it will make the price of altcoin increases (including the price of Ethereum).
Your mistake lies when you think that "when the price of bitcoin goes up, the price of the altcoin also goes up", that's where my thinking is wrong. because in crypto trading it cannot be guided by the movement of bitcoin because all altcoins have their own trends and movements. so my advice to invest in altcoins should not be guided by bitcoin but based on developmental information about altcoin itself.