I invite anyone to come up with 1 reason, just 1, why the legacy financial system will be using blockchain tech
You are absolutely right that blockchain tech doesn't provide the legacy system an advantage over a centralized database system that's better in almost every possible way.
If financial institutions want, they without the blockchain can set up a network enabling instant transactions from bank A to bank B, etc. They just don't want it because they thrive on the fees each hop generates for them.
Overall, regardless of whatever business it concerns, they all just use the blockchain hype momentum to raise capital and enjoy the free press that comes with it. Can't blame them for using the momentum in their advantage.