Maybe the point I was making was too subtle.
Look, it's not like the battery imploded or went supernova. It's deteriorating with time and use, just like every other battery is expected to do. The battery didn't catastrophically fail.
The fact that it reaches some magic voltage threshold where the i3 bricks itself, sure, I'll agree that can be a catastrophe for the owner, but it's ridiculous that BMW designed a system that allows it to get to this state. Can you have a "planned catastrophe" that every single i3 will go through unless the 12v battery is preemptively changed? I don't think so.
Again, the battery is decaying normally, predictably, and measurably, just like every other battery.
So back to my point about the super-important registration (it's not). If the system was actually monitoring battery health as it ages (it's not), the car would most certainly flash a maintenance warning to owner before ever getting to such a state (it doesn't).
FWIW, my 2013 Audi "required" battery registration, and that system flashes a "weak battery" message when it's time to swap in a transplant. If Audi can do that on an entry level A4 variant with an ECU that predates the i3, why can't BMW incorporate that same feature in a car that depends on the battery?
And FWIWx2, I neglected to register the Audi battery -- also an AGM type -- and instead connected a 12v charger and performed a hot swap, like for like. No ill effects suffered in the remaining 3 years I owned that car.
That's all I'm saying.