Building it isn’t the hard part.

Marketing it is. I actually thought about this idea for a long time, and did a design based off of the phonebook in iOS 11.

But marketing this application is nearly impossible, because it doesn't have any extra features. It just has phone numbers [contact details], and that doesn't excite users to spend time getting it set up.

So I concluded that this is something that has to be done from the OS vendor side. We need an extension to the CardDAV protocol, and we need vendors to implement it.

Indeed, building it is easy but getting people to update it is hard.

A great many people that I communicate with stopped using email regularly ages ago. For younger folks the means of communication are messenger apps and social platforms.

Keeping it sane is.