You said: Then it would be better to not have missionaries.
Except for the Great Commission (Go ye therefore into all nations....) you might be right.
There are people posting on this thread, claiming to be christian, that think salvation can come without faith in Jesus Christ. Some of them think that if unbelievers have never heard the gospel message, they will automatically go to heaven anyway...so, why would they want to have missionaries...it would mean that some would hear and reject, sending them to hell...if only they hadn't heard, all would have been ok. See?