All it has to do is calculate based off of current usage. It really is much more simple then you make it out to be. If you are on the phone and look at it, it will tell you how long you have if you keep talking on the phone until it is dead. As soon as you hang up, that time remaining increases based off of the now current usage (not on the phone, but still in non-standby mode). Once it goes into standby, this can be recalculated based off of low power mode.
I could easily program this in windows, All it has to do is calculate based off of current voltage usage, as it goes up, the time remaining is decreased, as it goes down, time remaining is increased. I would need to know the minimum voltage of the battery when the device shuts off. I am not an iOS programmer, so I would have no idea how to do this in iOS, but it would be easy enough to do in windows.
Now, I see no real need for this, but wouldn't hate it if they implemented it with an on/off switch. Just like numeric % of battery remaining, love it or hate it, there are people who want it back, and if it is implemented like it was in the past (on/off switch), no one is forced either way.