I have a short theory on why the Man in Black thinks the Island doesn't need protecting. On the surface it does appear that the Island needs protecting because various groups of people are always coming to it, destroying and corrupting as MIB would say. But I think MIB is referring to that without the meddlings of Jacob, the Island wouldn't need protecting because nobody would ever come to it. I think that MIB is under the impression that everybody who has come to the Island during Jacob's time has been by the hand of Jacob. So without Jacob's actions, the Island doesn't need to protected.