I don't think btc crash is good for alts. All alts will eventually be dragged down if the main coin, btc, that is paired almost every other coin with goes down as well.
That's right, and also don't forget that every other altcoin is "paired" with BTC in most people's minds. And I don't mean crypto enthusiasts or traders, but rather regular people who are only in the process of thinking on investing in this or that coin. Most of those people would abandon the idea of investing in any cryptocurrency if they saw Bitcoin crashed.