Well I didn't say he was as good a director as Spielberg and Scorsese. I do think he's better than Riddly Scott though.
Riddly Scott is a good director. When he gets a good script more often than not he makes good movies. The Martian was a good adaptation. The book was better but the adaptation was good. Alien, Blade Runner and a lot of other things he's directed were good too. But you give him a bad story he's going to give you a bad movie.
Exodus: Gods and Kings for example.
James Cameron is a writer/director. He not only directed Aliens, Terminator, The Abyss, Titanic, and Avatar(which was Sci-Fi "Dances with Wolves" and I liked it) he wrote them. James Cameron is the complete creative package. The movies he's involved with are created and shaping almost completely by him. So his movies are as good as they are entirely because of him. And almost all his movies are at least good.
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My point here isn't necessarily that you need James Cameron to make a good Alien movie or even a writer/director. My point is that Riddley Scott was never responsible for how great the Alien franchise was. Having Riddly Scott be in charge of the franchise is a problem because he doesn't actually know what a great Alien movie should look like or how to get there. Riddley Scott had nothing to do with writing in the original Alien. He just made a creature feature and it was pretty great. But I don't think anybody loved it for the directing. When Cameron came along with Aliens and created the concept of Colonial Marines. He turned it into a full blow Sci-Fi action/horror franchise. I think the Alien franchise needs somebody to do that again.
One of the problems with Alien: Covenant was that Scott had it written by
Jack Paglen(Transcendence) and
Michael Green(Green Lantern). That's obviously not the class of writer you need to make the Alien franchise what it was in the 90s. To make the Alien franchise what it once was you need somebody at the wheel that knows what they are doing from a creative standpoint. But as long as Scott's playing creative lead we're going to keep getting crap like this because he doesn't know how to reach that perfect blend of horror and sci-fi action that Cameron found with Aliens.
I'm sure somewhere out there is a great Sci-Fi novelist that's just dying to be that person to do that for him. Alien type books are not at all rare. Pair somebody like that up with a competent screenwriter and you've got something Riddly Scott can work with. The ones that come to mind for me are people like like Dan Abnett, Michael A. Stackpole, and Timothy Zahn but I'm sure there other even better people out there.
The problem is that I don't know if Riddly Scott realizes his limits enough to actually do that.