Well if you have a 51% attack you could control the network therefore easily destroying a blockchain. Bitcoin Gold got attacked recently and if it can happen to a currency that big, hypothetically if can happen to almost any currency (excluding Bitcoin obviously 
).
).Well in this case it's not anything like the blockchain being destroyed as much as it's about people losing money by being on the wrong fork temporarily. In this case and probably most cases it ends up being exchanges, since they're of course the biggest target.