This is flat out false, and misses the point altogether. First of all, plenty of systems have the ability to roll back transactions. Exchanges can and have dec

This is flat out false, and misses the point altogether. First of all, plenty of systems have the ability to roll back transactions. Exchanges can and have declared some transactions void if they were the result of market manipulation or extreme volatility, such as during the famous flash crash. Second, with a blockchain you need the other party to sign the reverse transaction. Either you have to trust that party, or a central and neutral authority with some sort of main key, to produce the inverse transaction. Once you have this, you have introduced a ton of trust and/or centralization into the system, in which case you could have just used a regular SQL database anyways.