A blockchain is just a blob of data. What matters is how you maintain that blob of data, and how you are using it. So if you are going to compare bitcoin to a centralised server, like Visa, bitcoin has a couple of advantages from an integrity point of view. One is that everyone can verify the transactions. There is not this black box called PayPal that you have to trust. There is no way for you to know what transactions have been going on in the backend, whereas everything is out in the open with bitcoin. Secondly, if I wanted to compromise some central bank’s repository, I just have to hack into a very small set of servers. Sure, they might be very well protected, they might have very good security measures in place, but it’s still a centralised system. So, in my opinion, it is easier to compromise than a system like bitcoin that has servers distributed over millions of computers to do some bad thing to the transactions like double-spending. Guess I should explain double-spending now..
Double-spending is one of the most common attacks people talk about when they talk about cryptocurrencies. To do that in a bitcoin setting, you’d have to have computational power that is more than the majority of notes in bitcoin, which is ridiculously high today. So if you look at it from just a security point of view, I can see some intrinsic benefits to using something like bitcoin to using a centralised system like PayPal.
There are other issues you need to consider apart from security, like performance and how many transactions you can do per second, how much time it takes for a transaction to go through, and if you can pay for coffee with Bitcoin. You can’t really pay for coffee because you need to wait for an hour for a transaction to be confirmed. So it’s not really clear-cut which way of doing things is better right now, but if I had to put my money somewhere, I would say cryptocurrencies are going to be on par if not better than centralised systems in the very near future.