> it won't (logistically can't) work when you have literally run out.
While I believe you're correct for the iPhone, that it won't work, it's actually not as impossible as you suggest. The NFC-capable BlackBerrys that supported the very early tap-to-pay with a phone had the concept of a default card, which could be programmed onto the secure element and would work even if the phone was totally dead (even if the battery was removed). The NFC field was enough power to boot up the secure element, just like it's enough power to run the chip in your bank card when you tap it.
Later phones dropped this support, as it took a bunch of engineering effort and customers largely didn't care. But if customers ever start demanding it, so they can totally stop carrying a bank/credit card, it is possible.
I suspect that the tiny amount of power you can vampire to make NFC work (which is why your contactless bank cards work as you explained) isn't enough for even the basic features we now expect from a smart phone as payment device.
So you'd have to message this very carefully, on top of the engineering effort, and my guess is that in reality "Reserve power" is always enough. If your phone "died" (screen turned off for lack of power) at the party, you have several hours after that when it can still do enough NFC to get on the bus home.
A lot of my friends get anxious at like 10%. Sure, at that point you should probably stop playing Candy Crush, but you're a long way from not being able to tap in to your train home if you stop. Power Reserve seems like a sensible choice to make you stop using the last dregs for frivolities.
We might see this again, as the Pixel 8 Pro has a system like this for UWB so your phone can be located by the Find My Device network after its power is drained.
While I believe you're correct for the iPhone, that it won't work, it's actually not as impossible as you suggest. The NFC-capable BlackBerrys that supported the very early tap-to-pay with a phone had the concept of a default card, which could be programmed onto the secure element and would work even if the phone was totally dead (even if the battery was removed). The NFC field was enough power to boot up the secure element, just like it's enough power to run the chip in your bank card when you tap it.
Later phones dropped this support, as it took a bunch of engineering effort and customers largely didn't care. But if customers ever start demanding it, so they can totally stop carrying a bank/credit card, it is possible.