If 80% of the hash power drops off, blocks will suddenly take 5 times longer on Bitcoin Segwit. On Bitcoin Cash they have a new feature called emergency hash adjustment which allows a fast response. So they will be fine.
The other thing to consider is that the capital will be hit hard. A lot is invested in mining rigs. OK, so this is someone else's problem, but the knock-on effects are sometimes hard to predict.
I've never understood why BTC didn't just agree to reduce block time to 1 minute or less like ETH.