Those people are in the app because of the social features and the feed in the first place. The messaging features were built on top of the platform.
Requiring companies to make and maintain a separate app entirely if their product has messaging features is an unreasonable requirement. If someone has such a strong self-control problem that they can’t message someone without becoming addicted to the feed, they shouldn’t be involved with the platform at all.
Just exchange emails, phone numbers for SMS, or any other type of communication. I seriously doubt that your friends are only able to communicate through exactly one communication channel and it happens to be Instagram.