The trouble with that argument, which rests on the hypothesis that Scorsese's earlier work is of such power and profundity that nobody else could equal it, not even Scorsese himself, is that it's nothing more than fanboy hyperbole.
I'm going to say something which won't be popular with the diehard Scorsese fans here but I feel it needs to be said.
I think that Scorsese may be great with a camera but he has yet to make a great movie. The overwhelming majority of his films suffer from very poor screenplays and very inconsistent direction. Agreed that most of his movies have individual shots of great power but Scorsese has always been unable to sustain that level throughout the whole film. Of his movies only After Hours and Goodfellas even approach greatness, IMO. Last Temptation, although not a great film, was at least a heartfelt one. But the rest of Scorsese's oeuvre simply doesn't hold up. There may be great individual moments in his movies but the movies themselves are not great.
Anything else? Well aside from the fact that the screenplays are frequently botched his films are usually thematically incoherent. For example it's not at all clear what the point of, say, Raging Bull is. The invariably alienating protagonists at the core of his films represent another obstacle to the claims of greatness made by his fans. It's hard to get worked up by any filmmaker who can't create the kind of interesting and sympathetic characters that viewers recall with fondness. And the director's infatuation with the sordidness of his characters is so rarely balanced by any other dramatic shading (see Casino) that they too often become boring and tedious.
Scorsese's other limitation is that thematically he hasn't shown much progression. Yes, he's tried different types of movies which on the surface sounds like a plus. Except that when you watch them the results are almost always disappointing, mediocre - in short - unremarkable. I mean, New York, New York is not going to go down in history as one of the great screen musicals, anymore than Cape Fear will go down as one of the great remakes, or Age of Innocence as a period romance whose themes transcended its genre. It's ironic that whilst Scorsese is well known as a fan of old Hollywood his movies consistently fail to come anywhere near the greatness of the directors he so admires.
In recent years he's moved into the big budget realm attempting to merge his urban street sensibility with, variously, an historical epic, a Hollywood biopic and another gangster movie, this one a remake. None of them seem likely to be remembered as definitive examples of their genre and after all isn't that the standard by which we should be judging a 'genius' by?
