Trust me this is not exactly the case. The drop in the price of Bitcoin greatly affected altcoins. So you find a situation where alts that were bought at certain prices and because there was a drop in the price of BTC, the dollar doesn't reflect the amount it was bought
I think what OP means is that if there is a Bitcoin downtrend Altcoins with potential to grow are cheap and we can buy some of them and hoard. We all know that Altcoins are dependent to Bitcoin that is why when there is price drop of Bitcoin we can say that it is good news for Altcoin investors. As a bounty hunter I can always have tokens from the bounty programs and I can say that when there is BTC plummet my holding sucks though I get it all for free just by investing my time and effort so it is still good because I am still hoping for a bounce back.