How exactly is Apple supposed to know that you have switched phones? Any way I can think of requires privacy intrusions that make me shudder. However, they do

How exactly is Apple supposed to know that you have switched phones? Any way I can think of requires privacy intrusions that make me shudder. However, they do have UI on all of their iMessage programs, and on the website that you can get to from any web browser (on any brand of device) for your AppleID/iCloud account that can remove your phone number from your AppleID at any time. Then, after some minor propagation time, messages from Apple users to your phone number will no longer be routed to your iMessages account, and will instead be sent as SMSs. So you have an option to tell Apple at any moment, even after you have switched, that you no longer want that number attached to iMessages. How exactly is that hijacking?