Contact information should belong to the person it is about
Your phone number is a fact about you. So is where you live, and how to reach you at work, and which of those you would rather a particular person did not have.
Now count how many copies of those facts exist that you have no control over. Every phone that ever saved you. Every exported spreadsheet. Every group chat where someone pasted your address so the delivery would arrive. Every card list, every class roster, every “can you send me everyone’s details” email that got forwarded twice.
You did not agree to most of those. You cannot see any of them. You certainly cannot correct them — and when one of them is wrong, the consequence lands on you. The card does not arrive. The invitation goes to the old address. Somebody concludes you moved without telling them.
The assumption underneath
This is not anyone behaving badly. It falls out of a design decision so old that it stopped looking like a decision: contact information is something other people keep copies of.
Every address book ever built assumes it. Paper ones did. Rolodexes did. The one on your phone does, and so does every personal CRM built in the last decade — they are all more sophisticated ways to maintain your own private copy of facts about other people.
Once you say it plainly it sounds obviously wrong. Zoe knows her number. She knew the exact day it changed. Somehow the burden of discovering that falls on forty other people, separately, each through their own small failure, months apart.
What we think is true instead
Contact information is not yours to store. It is theirs to publish.
You should not hold a copy of Zoe’s number. You should hold a connection to Zoe, and her number should be one of the things that connection carries — current because it comes from her, and present only for as long as she wants you to have it.
That inverts who does the work and who holds the control, and both of those inversions are the point. She updates once instead of forty people updating separately. She decides what you get instead of you deciding what to write down.
Why it has to be mutual
Here is the part that makes it more than a filing improvement.
If contact details flow from the person they belong to, then they flow both ways by construction. You always have theirs. They always have yours. Neither of you has to ask, and neither of you has to be the one who remembers to send the update.
That is not a better address book. It is a different relationship with the people in it — one where staying reachable is not a thing you maintain, but a thing that is simply true because you are connected.
Nobody decides to lose touch. It happens by default, one silently wrong record at a time. We think the default is worth changing.