As far as I know when bitcoin goes down, altcoins also go down because almost all altcoins follow bitcoin. This is the reality that I have observed so far. although there are altcoins that continue to rise when bitcoin goes down, this is usually a new project although rare.
Although not all altcoins follow Bitcoin, I agree with your assertion that the majority do. There are many examples of when BTC went up but not all alts, and when BTC went down but not all alts. If they were, trading altcoins would be far easier, but they are not exactly proportionate to one another.
Since trading altcoins is more profitable than trading bitcoin, even if we can accurately forecast the price of bitcoin, we cannot guarantee that the altcoin we have invested in will increase in value. Nevertheless, we have made some accurate predictions, so we will continue to invest in altcoins rather than Bitcoin.