The 12 V battery in our i3 manufactured on 1 August 2014 hasn't failed yet. I bought a replacement battery in April, 2019, but haven't needed it yet. Like your i3, ours hasn't been driven very much (less than 12k miles). Maybe that's the secret to a long 12 V battery life.
I haven't found a good way to detect a weak 12 V battery while it's installed. The voltage at the auxiliary power port is almost always the DC-DC converter's output voltage of ~14.3 V. For a few seconds before the DC-DC converter turns on, I can watch the voltage of the 12 V battery drop to as low as 11.90 V before the DC-DC converter turns on. This low voltage alarmed me at first, but it's been this low for over 3 years. The service manager at the local BMW dealer told me that this is normal for an i3 when its electronic modules are booting.
2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE