Even if this attack is a success, I don't think BCH value will be down to the worrying level. They will simply fix it and everything will be same as before. People are not rational nowadays, altcoin like XVG, BTG, EOS, etc. still hold high value despite multiple attacks and/or bugs in their network.
some things aren't fixable!
for example when you have 32 MB blocks and the whole reason why your altcoin exists is to have a bigger block, you can't just fork back and turn it to a smaller size block again.
or when your coin (BTG) is successfully attacked and is easy to wipe its blockchain with a 51% attack you can't do much about it. and it has lost value. it was 0.3
BTC at some point and now it is 0.003
BTC.
or when your blockchain is more than 1 TB (ETH) you can't do anything about it. it is your blockchain and you can't discard it, you are already centralized.