I think if bitcoin crashes everything will obviously fall. Including eth. It will stay at that point for about a week until it settles. Even if that settlement point is at 0. Then you would see a rapid rise in eth and since eth would be the main trading pair the altcoins would follow. Everything would survive in the end. But honestly the only way btc crashes like that is if a unfixable bug is found in the code. But if it crashes there would be that turning point to Ethereum.