What? Are you serious?
Taking a mandate from Labour members means very little as they are a tiny proportion of the population who vote Labour. Second, you're assuming he would win another mandate from them which may or may not be true once they see he exercises zero control over his actual party.
Under Blair, Labour won three elections in a row. Under Foot and Kinnock who were both more charismatic leaders than Corbyn they failed to do so even when facing some of the most unpopular governments in history.
History ain't on his side, he has poor relations with the media, zero support from his own MPs and you can tell what the Conservatives think of him by the eagerness with which they agree a quick GE is required.