The point is that in your proposed system in which the customer keeps their local bank account number when moving banks, there will still be places where the customer will have to change their number - i.e. everywhere where their IBAN is stored. The IBAN cannot remain the same, because international institutions need to know which institutions they are dealing with. Not all institutions are legally allowed to or want to transact with all institutions.
So the added layer of complexity is that customers will have to change their number in some places but not in others. And banks will have to have a system of ensuring they match the right IBAN with the right local account number. Whenever this fails, there will be problems such as delays or money being deposited in wrong accounts etc.
This is unlike DNS over IP, which is universally/internationally agreed protocol which has 100% coverage. But even in that system, whenever you change your DNS settings, it takes a while to propagate, and in this time there are all sorts of weird errors. Cat picture websites can tolerate those sorts of errors. Financials institutions should not.
Can't Finland register the whole FI prefix as institution "Finland"? If other countries' institutions need to know the actual institution, well... it wouldn't be the first time two countries had conflicting requirements, and politicians had to get together and yell at each other a bunch and suddenly a law was passed to solve it.
You mean universally/internationally agreed protocol and maybe a set of laws like e.g. IBAN?
The bank numbering system was indeed a static routing code, like phone numbers were in old landlines. Why the former can't be changed but the latter could?
Of course the IBAN system can be changed. It will just require a revision of international treaties, which will require dozens of countries agreeing to it. Finland has little control there.
So the added layer of complexity is that customers will have to change their number in some places but not in others. And banks will have to have a system of ensuring they match the right IBAN with the right local account number. Whenever this fails, there will be problems such as delays or money being deposited in wrong accounts etc.
This is unlike DNS over IP, which is universally/internationally agreed protocol which has 100% coverage. But even in that system, whenever you change your DNS settings, it takes a while to propagate, and in this time there are all sorts of weird errors. Cat picture websites can tolerate those sorts of errors. Financials institutions should not.