The “Sin Unto Death” is maximum punishment from God when He removes the believer from earth by physical death. It actually entails a maximum amount of time in reversionism (“backsliding”), out of fellowship, and useless to God. God calls the believer home so that sin doesn’t keep piling up.
Removal from earth is a demonstration of God’s grace and love toward every believer, even those who have forsaken God. We are not faithful, but God is faithful and will not let us suffer or be tempted above that which we are able (1 Corinthians 10:13).