This event has been and is EXACTLY the reason to avoid Bitcoin Cash (BCH), the coin of the forked chain and initiative of Roger Ver. Immutability is the biggest selling point of blockchain technology. There was some concern of a 51% attack in regards to Bitcoin's blockchain during the early days. But as the depth grew and the ecosystem matured, the possibility of a 51% attack become more and more remote. Vitalik Buterin pulled off a rollback with Ethereum's chain in its early days after a hacker siphoned off a large amount of Ether in a child app [https://steemit.com/ethereum/@squirrellywrath/ethereum-and-life-imitating-art]. We all had to hold our noses a little during the rollback. Now there has been a successful 51% attack on BCH (to supposedly reverse a hacker's theft during a code update). Sorry, but the increase of block size which toughened barriers to entry is what gave these miners the ability and power to pull off this 51% and reverse the "questionable" transactions. BCH is a power grab. The people's currency is Bitcoin.
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read:https://cointelegraph.com/news/two-miners-purportedly-execute-51-attack-on-bitcoin-cash-blockchain
" two miners with majority control of the network — BTC.top and BTC.com — performed the attack in an effort to stop an unknown miner from taking coins that were sent to an “anyone can spend” address following the original hard fork in May 2017."