Once the rider is thrown off the motorcycle, the motorcycle will eventually crash, imparting force onto the phone while it is still attached to the motorcycle.
If the phone is in the pocket of the rider at the time of accident, this can be detected using the proximity sensor or nominal accelerometer data crosschecked with GPS velocity, or even capacitive sensing.
EDIT: There are only three possible states before the crash. Either the phone is in the rider's pocket, or it is mounted to the vehicle, or it is held in the user's hand. If the phone is in the rider's pocket, it is going to stay there until the rider feels a strong impact. If the phone is in the rider's hand, then the phone should turn this off completely because it's far more likely to slip. If the phone is attached to the vehicle, it should only register a crash if it begins simultaneously with the phone being disconnected to the vehicle (if it is to begin with).
The fact that the three may be separated is irrelevant, because they will be separated after the crash begins. As long as the phone mount is sturdy enough not to separate under normal conditions, this is fine. If it is not, then a false positive is much more likely than a true positive. And a crash sensor with more false positives than true positives is actively harmful.
This only applies to two wheeled vehicles, obviously. For four wheeled vehicles, the way it's working right now is fine. It is pretty trivial for a phone to make the difference between a car and a bike.
It doesn't sound like you've ever seen a crash.
If the phone is on the pocket it can easily get separated from the ride before the rider feels a strong impact.
I'm not sure how you think the phone should figure out if it is disconnected from the bike.
While it may be possible to improve crash detection. I don't think any of your suggestions come close. And generally will make things worse
Why? Apple's implementation is clearly suboptimal. Motorcycle crash sensors aren't new, and they work differently to this for very good reason. Sensing whether the phone is on or off the motorcycle at the time of the crash is a really basic feature, because phones fall of bicycles and motorcycles almost as often as there are crashes to begin with.
A feature with that many false positives can be actively harmful. It's not responsible engineering to ship it. Especially if you lock it to specialized hardware, then you have no excuse not to go all the way and at least have a proximity sensor at the back.
Why would we assume the only possible states for the phone in a motorcycle crash would be attached to the motorcycle or attached to the rider? It's perfectly within the realm of possibility for the rider, cycle and phone to all be separated from each other.
And if its sitting in a cupholder in a car? If the phone is in loose in a saddle bag? In a backpack on the floor of a car? In a backpack on their back, out of range of a proximity detector? If the motorcyclist hits the phone on the way down and the phone goes tumbling and ends up hitting the ground in a way indistinguishable from the situation OP had?
Apple clearly chose to accept false positives in order to avoid false negatives. I'm pretty sure their engineers and lawyers have given this more thought than you have from your armchair in the one hour since this was posted.
