I agree with the opt-out for the registration lock, but i don't see why the expiry is an issue. The registration lock is lifted after a week of inactivity of *all* devices.
Reregistering the number (w/o pin) would immediately clear all accociated data, AND notify all peers that the security number changed. (That indeed has it's own ux-issues, but that is true or worse for all other messengers aswell)
When i think about this, making the pin permanent would 1/n