The phone number link means forward security isn't possible. If ever the encryption is hacked, all your messages could be forfeit by anyone who's simply kept the encrypted data.
Can you elaborate on that? Obviously the phone number has privacy implications, but I don't think it can be used to decrypt messages. In the signal protocol, encryption keys are exchanged using ECDH (so wiretapping doesn't work) and periodically rotated (so even knowing the encryption keys at a certain point doesn't let you decrypt messages after that).
So what is the reason for doing it that way?