Keybase also has the system where you can approve a new device from another. Why can't we have the same thing w/ Signal? Maybe tack on the PIN to make it a bit more secure.
If you backup the Signal data on the old device and restore it on the new one, your safety number won't change. This is the proper way to securely migrate to a new device.
I think that's a serious backdoor for a coercive attacker. They only need access to the original device once and can then assume the identity of the contact on another device.