Preachers are needed to preach.
God only revealed his message to his Messengers, his Prophets, the last, final one of whom was the greatest of them Prophet Muhammad (PBUH).
Unless you are one of those twisted individuals who claim to have received messages directly from God, you just like most other human beings from any era need to just come to terms with reality. Allah SWT revealed his messages to his Messengers, the last, final one of whom was the greatest of them all, Prophet Muhammad (PBUH).
Allah SWT's messages as revealed to Prophet Muhammad PBUH are known as the Quran, that can't be altered even if all of humanity today and all humans that are yet to be born were to join together and try to alter it. Allah SWT promised it in the Quran that he will protect it from corruption and sure enough he has and he will continue to do so. So Muslims need not worry about that.
Zakir Naik is just a preacher, but a fine example of that. He preaches and spreads awareness among non Muslims, but also among Muslims. Nothing wrong with that.