The reversibility of transactions is often brought up but I don't think that is the main differentiator, or any at all really.

First, both cryptocurrencies and traditional finance systems have chosen to and have chosen not to reverse or freeze criminal transactions in the past. Decentralized blockchains have chosen to do so at least once [0]. The traditional finance system has chosen not to many times and even large sums [1] [2]. It really comes down to how much is stolen and how much influence the victim(s) have.

No, instead I think the main difference is the speed of transactions. Some blockchains operate within milliseconds while bank transactions take days. Even instant transfers are actually settled on a way longer cadence in reality.

That means attackers can do a fast sweep on blockchains, which is not possible with bank transactions because everything happens in slow motion and humans have way more time to react and step in.

[0]: https://en.wikipedia.org/wiki/The_DAO [1]: https://www.turkiyetoday.com/turkiye/german-bank-e100m-disap... [2]: https://en.wikipedia.org/wiki/Bangladesh_Bank_robbery

I imagine it's possible to design a blockchain where transactions are 1) artificially delayed; and 2) can be blocked by anyone who has the key.

For example, take Bitcoin, except that each transaction must be posted twice to be considered to have modified the ledger between 7 and 14 days apart as measured by the number of intermediate blocks as a function of the difficulty requirement. If a "blocking" transaction makes it to the blockchain in between, then the transaction is deemed to have been cancelled. A "blocking" transaction must merely demonstrate ownership of one of the transaction inputs.

If you hold large funds, you could keep a remote monitoring system somewhere that is empowered to block your own transactions.

The blockchain design could be enhanced so that a blocking transaction proves ownership of the original inputs but cannot sign a transaction itself.

Failure mode: someone trying to legitimately block a transaction could find themselves unable to get their blocking transaction mined. But in practice they'd just up the transaction fee, and game theory should take care of it.

This was just a thought experiment. A point that comes to mind is that given this is possible, what you describe isn't an inherent property of cryptocurrency, so if there's a benefit to having this property then it could be done.

The main reason it is a differentiator is that centralised money understands that its purpose is to be a system of social credit, so reversibility of transactions is just an acknowledgement that there is enough evidence to prove that this particular individual owns this particular sum, so it should go back to them. It is actually a much better and more honest system than the blockchain, because it doesn't treat money like a tangible resource in and of itself that can be "lost" which would be a much older and dumber way of doing things.

It’s not always a choice. If value has been transferred onwards (ultimately in the form of e.g. cash withdrawn or something equally hard to trace, like physical goods handed over to a buyer), simply “undoing all transactions in a database” isn’t without victims either.

Whether undoing transactions or not is an option strongly depends on how fast value is allowed to move. Sometimes being slow is a feature as much as it is a bug.