Whose existing banking system? The Chinese one? The Japanese one? The Egyptian one? The Indian one?

The Ancient Roman one.

The one most people use. SWIFT.

And yeah, blah blah sanctions, Russia, Iran, like most of the world's business is happening there and like the crypto Ponzi schemers really care about the "under-banked".

How's swift suitable for micropayments?

SWIFT isn't remotely suitable for micropayments. Most banks have to go through a correspondent bank that is going to charge at least $5 per transfer.

Ok, then use anything else or get banks to come up with another one.

How, pray tell, would a cryptocurrency help here? You have all the crypto disadvantages (either untested or slow or scams or boiling the planet...) plus the same issue with adoption. We already have banks and they already have bank accounts. We have SWIFT, SEPA, etc. They know how to talk to each other. Get them to modify an existing protocol or make a new one for microtransactions.

It's a super hard task but with crypto you still have that super hard task (adoption) plus crypto itself (a million drawbacks).

It's not scientific and it's not engineering.

You solve a problem with the simplest equivalent solution, no more.

Crypto is like God. It gets sprinkled over every discussion. There's a reason science doesn't go: "A equals B plus God" to prove anything.

The “ancient Roman one”? Enlighten me.

Any. Every. Whatever the user has. Banks already figure this out amongst each other - the browser just needs to convey an IBAN (or other format), currency and amount to the user's bank and let it be their problem.

Exactly. I proposed SWIFT and people latched onto to it to criticize when it was just a detail.

You put it better, just have any layer on top of IBANs. 0 need for crypto.

However, the API should also be able to support crypto payments, by getting a crypto address and then forwarding the request to e.g. a local instance of Metamask, instead of my bank.

Perhaps if the user has multiple bank accounts linked, they would have to choose one, to avoid leaking to my bank that I'm making a crypto payment.

Of course, a crypto transaction has a significantly lower chance of being compatible since there's no foreign currency team working at the bank - if the website wants a payment in BitWhatsits and my primary wallet is in DOGEMONEY 20000!!!!! then it's up to the user or at least the extension to figure out the routing. Probably, the website would offer a different button to pay with each supported cryptocurrency, so the user would choose. They'll be accustomed to that, being a cryptocurrency and all.

I don't mind that. People should be allowed to shoot themselves in the foot with their favorite Dogecoin.

Different ones for different markets.

A working standard could simply define "seller endpoints", "buyers bank endpoint", "supported payment protocol" and a message passing system and let the parties figure it out.

Not global, but few things are in the world of finance.