To buy Altcoin coin you have to rely on Bitcoin. There is a possibility of dumping altcoin when BTC Dominance increases. And when bitcoin suddenly dumps, so does altcoin. BTC pumps altcoin even when it is fixed at a specified price.
You have a point because most of the altcoins always follow the bitcoins movement. There are altcoins that also increasing when bitcoin going up. But I think still be careful to choose the altcoin you want to buy and invest.