Kyrie Irving is 6'3. So is Sexton. Kyle Lowry is 6'1. Lowry is #2 in real defensive plus/minus. #1 is Marcus Smart, 6'4. Tyus Jones, 6'2, is 5th. Chris Paul, only 6 feet, is fourth and has generally been considered a solid defender. Sexton is recorded as 6'3. I get that he's a about 6'2 with shoes in reality, but there's a lot of fudging of the heights of many NBA players.
Feel for the game is legit. Basketball IQ is legit.
Alexander is 64th in Real Defensive Plus/Minus. Matthew Dellavedova ranks higher.
Sexton is 97th, dead last, just behind Trae Young, Jordan Clarkson, and Spencer Dinwiddie.
De'Aaron Fox (also 6'3 - so another guy that's too short) is 23rd this year and positive on defense. Last year he was 75th.
Maybe things won't work out with Sexton, but look at the leap Fox took with heard work and good coaching. He was Isaiah Thomas level bad defensively last year. This year he is average at least - no longer a major leak in the D.
Sexton is far from short. He's short if you want to move him the to 2 and there are legit concerns about his assists, etc. No agreement that he needs development. He also is not scared of hard work. We need the proper coaches and mentors around him in the offseason so the Cavs can do everything in their power for him to become De'Aaron Fox 2.0. That will take some work, but he has the tools.
He also has a wingspan that is about on par with Fox. 6'7.25" (as measured at combine) and it may have gotten a bit longer since he's still growing a bit.
Also should be noted that Sexton has a longer wingspan than:
Kyle Lowry
Kyrie Irving
Steph Curry
Chris Paul
Trae Young
Mike Conley
Tony Parker
If Sexton doesn't have the measurables to play, neither did they. None of those guys had a wingspan above 6'4. Sexton has 3 inches on them and an 8'2.5" standing reach. Kyrie Irving's standing reach is 8'2". Fox is slightly better at 8'4".
The arguments on IQ, seasoning, etc are fine. Height is a BS argument on Sexton. He has the size, reach, and wingspan that matches or exceeds some of the best at the position. He has the physical tools and work ethic to succeed. Let's see if he can put it all together.