That's certainly true as well. Not only is replacing those legacy systems expensive but expediting the transfer process would actually be against the banks' int

That's certainly true as well. Not only is replacing those legacy systems expensive but expediting the transfer process would actually be against the banks' interest (because they're still able to use that money while it's in digital limbo). If banks were only allowed to subtract the amount from the sending account as soon as they got confirmation from the receiving party that the money arrived and was added to the receiving account this would incentivize them to make this process as fast as technically possible.