Scorsese has an op-ed in the New York Times where he elaborates on his comments that Marvel movies are not cinema. He does not back down.

Many franchise films are made by people of considerable talent and artistry. You can see it on the screen. The fact that the films themselves don’t interest me is a matter of personal taste and temperament. I know that if I were younger, if I’d come of age at a later time, I might have been excited by these pictures and maybe even wanted to make one myself. But I grew up when I did and I developed a sense of movies — of what they were and what they could be — that was as far from the Marvel universe as we on Earth are from Alpha Centauri.

For me, for the filmmakers I came to love and respect, for my friends who started making movies around the same time that I did, cinema was about revelation — aesthetic, emotional and spiritual revelation. It was about characters — the complexity of people and their contradictory and sometimes paradoxical natures, the way they can hurt one another and love one another and suddenly come face to face with themselves.

It was about confronting the unexpected on the screen and in the life it dramatized and interpreted, and enlarging the sense of what was possible in the art form.
Some say that Hitchcock’s pictures had a sameness to them, and perhaps that’s true — Hitchcock himself wondered about it. But the sameness of today’s franchise pictures is something else again. Many of the elements that define cinema as I know it are there in Marvel pictures. What’s not there is revelation, mystery or genuine emotional danger. Nothing is at risk. The pictures are made to satisfy a specific set of demands, and they are designed as variations on a finite number of themes.

They are sequels in name but they are remakes in spirit, and everything in them is officially sanctioned because it can’t really be any other way. That’s the nature of modern film franchises: market-researched, audience-tested, vetted, modified, revetted and remodified until they’re ready for consumption.

Another way of putting it would be that they are everything that the films of Paul Thomas Anderson or Claire Denis or Spike Lee or Ari Aster or Kathryn Bigelow or Wes Anderson are not. When I watch a movie by any of those filmmakers, I know I’m going to see something absolutely new and be taken to unexpected and maybe even unnameable areas of experience. My sense of what is possible in telling stories with moving images and sounds is going to be expanded.

So, you might ask, what’s my problem? Why not just let superhero films and other franchise films be? The reason is simple. In many places around this country and around the world, franchise films are now your primary choice if you want to see something on the big screen. It’s a perilous time in film exhibition, and there are fewer independent theaters than ever. The equation has flipped and streaming has become the primary delivery system. Still, I don’t know a single filmmaker who doesn’t want to design films for the big screen, to be projected before audiences in theaters.
I think his general concern about the lack of opportunities to see a variety of films on the big screen is right, but he's wrong about Marvel films lacking variety or character complexity, at least relative to typical Hollywood blockbusters.