> if you need more entropy then add another digit.

That would break so much as most of the message formats and batch files use fixed width fields. It's probably the same for the databases underneath. Changing to alphanumerics sounds nice but the check digit algorithm would have to change from Luhn to Luhn mod n. More breaking changes.

Banking systems are one of the ultimate forms of legacy, which is why most of the security additions have been bolted on. Just look at 3D secure - they added an entire subsection to the message format and it was still just another single factor auth method that nobody wanted and everybody hated.