I disagree, the main things that prevents Sexton from being a good defender are his instincts, awareness, and lack of defensive range. Being "overly aggressive" is a big part of the problem and his "high effort" is often confused with a lack of defensive IQ. He's also best at the visible parts of defense, for example playing tight defense on the ball, but is extremely lacking in the other and likely more important areas of defense such as covering ground off the ball, making rotations, and executing coverages correctly. Don't get me wrong, on ball defense is important, but it is way overvalued in the entire scope of defense. When people get optimistic about Sexton's defense, it's always about his effort and what he do directly on the ball. No one ever mentions the other stuff, and that's the stuff that makes him a soundly negative defender.
You could say much of this is learnable....but it's kind of like saying shooting can be learned and developed. Some players are able to pick up and learn things better than other. In the same way that some guys have a natural touch and feel for the game, there are things on defense that come naturally and we know by now that Sexton is not a gifted defender. He can learn from guys like Lowry and FVV....but those guys have somethin innate that he does not have. Like I said, he can improve, but he will probably always be bad and that's true no matter what position he's playing or what his defensive assignment is. Most smaller guards are, and I don't think he has the natural ability to overcome those odds. Not everyone can be a defensive savant.