Depends on what reasons the BTC would crash. But because BTC is the main pillar for crypto, any disturbance to it will have ripple effects (no pun intended) on the market, that's natural.

But if we are to think that BTC had many crashes and still alts still survived long term but also highly thrived and increased in numbers.

So I wouldn't say a crash would destroy alts but of course, would hurt everyone.