The Messenger is coming—must come—because the LORD God has promised, and He is unchanging and always faithful no matter how unfaithful His people may be.
The Israelites had taken the Covenantal promise and the language of separation and interpreted them to mean the message of salvation and restoration was meant for only them. But this is counter to the reality of the Scriptures.
The full effect of the Law had been visited upon God's people, but now the LORD will remember His people and return them to the land of promise and to Holy Jerusalem.