Which of the world’s major religions, if any, do you expect will disappear first, once Maitreya begins his work openly?

None of them. It is not Maitreya’s task to stimulate the destruction of any of the major religions ― quite the contrary. His main work, in the first place, will be to stimulate economic, political, and social change. When he does eventually inaugurate the new world religion based on initiation, its adherents will be drawn, as is already the case, from all religions.