That's all fair. However, two points left out:
1. As we all know, players don't put on any weight after the age of 20... and as far as measurables are concerned, Kawhi definitely has that freakish reach on him, plus those hands, I'll give you that. But Josh has already measured out taller w/o shoes than Kawhi did at the combine, and it wouldn't surprise me if he grew an inch to be a true 6-8. And while my initial comment was obviously steeped in sarcasm, you have to think he put on some muscle mass in his year at Kansas. I've seen first hand the insane impact they have on these guys. Just look at a guy like Withey from his junior to senior year, or LaGerald Vick from last year to this.
2. Physical freaks turning into star players doesn't mean players without freakishly long arms can't turn into star players. This league is filled with players that are of average measurements (relatively speaking, in reality, all of these guys are physical freaks) that are also superstars. And I have to say, I think you are seriously underselling Jackson's skills. The kid is putting up better numbers across the board in his freshman year than Leonard did in his sophomore year, and he's doing it against better competition while being asked to guard out of position.
I think a lot of people forget what Leonard looked like coming out of SDSU. We all know what Leonard became, and people laugh at teams that passed on him. People thought he could be really good, but how many saw him becoming a 26 ppg guy you could run the offense through while he guarded the other team's best player? Maybe a couple... but not a lot. Hell, even fewer thought he could become a decent shooter, let alone a guy you can't leave open.
When I say Josh has Leonard potential, the key word there is potential. I'm not saying he will become Kawhi, but all of the tools are there. If he ends up in the right situation, it would not surprise me one bit if he reached that level in 6-7 years.
Even with all that, I agree Ball is the best prospect, and I'd even consider Fultz or Smith over Jackson. I do not agree that Tatum is better, though I can understand the reasoning. At the end of the day, this draft is going to come down to fit, I think. A team like Philly is definitely taking a PG. The Celtics... probably a PG. The Suns, I would think, are a wing team. The Magic strike me as a PG team. I think the Wolves go wing, if they are concerned about LaVine, PG if they don't think Dunn is the answer.
All that said, it's kind of dumb for us to have this argument. We can't know the answer for years, and the Cavs have no shot at any of these guys.
(Ugh, I feel like Eli's Eyes the way I'm caping for Jackson
.... he's 100x better than Perry Jones though ha)