There's nothing technically stopping it from being used in other countries though, Hong Kong has implemented the same system with same level of success as Japan.

> You can't use it internationally, unlike contactless EMV.

Technically it's incorrect. Visa/Mastercard works internationally, I have a RuPay card from India and it didn't work internationally until recently. So it depends on the reciprocal agreement between networks - https://en.wikipedia.org/wiki/Card_reciprocal_agreements

Yes HK has its own stored value card, but you can't use Octopus in Japan, can you?

Also I'm sure you'd agree it's easier to trust something issued internationally if there was some element of online verification to it (admittedly probably not much of an issue for transit applications).

This is where Apple and Google step in, you can load Octopus pay on your phone and start using it like a local. Apple even notifies you about Suica as soon as you land in Japan.