That's exactly what the system contact picker does (it gives the app access to the selected contact, without giving the app access to other contacts or to futur

That's exactly what the system contact picker does (it gives the app access to the selected contact, without giving the app access to other contacts or to future changes to the selected contact). There are apps out there where exposing all your contacts is a desirable behavior, though, so they can't get rid of the "read all contacts" permission.