I’d hate to run out of battery or accidentally break my phone and lose access to something as basic as an identity document or my main means of transportation, at the same time as having lost access to call for help.
I’ve boarded trains and bought items in Japan with a “completely dead” iPhone using the power reserve feature where Express Transit cards remain available. It actually works really well, and means I don’t have much fear using my phone for those things in Japan.
Granted, ID cards aren’t like this, but there’s no reason why a system couldn’t be added for them.
You could also cut that chip out and put it on a cute keychain. :)
Mobile Suica is far more useful than a physical Suica, with features like being able to review transactions on your phone, refill anywhere, add passes anywhere, and buy Suica green tickets, commuter passes, etc while not at a station terminal.
Good luck inserting your cute keychain into a machine for charging or performing basic Suica functions. You’re also completely locked out of commuter passes if you try that, because the machine needs to write that data physically on the card surface.
Pass the dead phone over wireless charging, enough juice passes into it for it to reply with your default cards.