Tatum has the least upside of anyone in the top five. Please don't let it be him
The problem is that when determining upside, most of you guys just look at athleticism and age and that's it. If the kid can run and jump like crazy and he's young, then boom - certain future superstar!!!
But lets look at some of impressive players that have come through the NBA over the years.
Tim Duncan was never a standout athlete - his game was based on intelligence and skill, although his size helped too...and that's why he was able to keep putting up productive seasons well into his late 30's.
Paul Pierce was never a standout athlete. He wasn't a bad athlete or a below average athlete, he just wasn't a standout one. His game was based on size (for his position), strength, skill, and Basketball IQ. Even as he aged and lost a step or two, his legendary footwork allowed him to create the space he needed to get shots off against some of the NBA's most elite defensive players, on a nightly basis.
John Stockton and Karl Malone are two other examples. Malone was a good athlete, but he wasn't running around hitting his head on rims, or running up and down the court like Allen Iverson. He was a solid athlete, but mainly relied on his skill and strength to be productive, and that carried him to something like 20 years of NBA success - same with Stockton.
Melo is another - he doesn't seem to have the drive or aggression he once did, but when he was in his prime Melo was arguably THE most devastating and versatile scorer in the NBA. He could not be defended by anybody, period. Yet he was never super athletic - just long, strong, and graced with in credible skill and footwork.
Two modern day guys heading in a similar direction are Jabari Parker and Gordon Hayward. Neither has Wiggins-like athleticism, and yet both are become very good young players based on the fact the fact that they have such good size and skill at their position.
Kobe and MJ as they got older late in their careers had a mere fraction of their former athleticism - and both have gone on record saying that in order to remain effective in their older ages, they needed to beef up and get stronger, train up and get smarter, become more skilled - because when you can't depend on athleticism for an edge most nights, you have to be smarter and more skilled then the next guy.
That is Tatum in a nutshell. He's not as athletic as guys like Wiggins or Brown or Jackson - but he has a natural offensive talent that is instinctive and cannot easily be taught He simply knows how to score the ball - straight up.
Now not all of those guys I just mentioned were ever considered top shelf defensive players. Melo is always mocked for his defence. Parker's defence is poor. Hayward's defensive is average at best. Pierce was seen as a poor defender early in his career, but seemed to actually get better as he got older due to sheer Basketball IQ. And similarly, Tatum may not become an elite defensive player - or even a good one. But I'm confident that he can be a solid one (much like Gordon Hayward has become) without too much trouble, once he gets more experienced.
I look at guys like Melo (who has never averaged under 20 PPG in his career) and Paul Pierce (who out-dueled Lebron on multiple occasions) and Gordon Hayward (who's really coming in to his own this year) - then I look at the "Tatum lacks upside" argument, and I get confused. What is upside? Because if I draft a guy who has the potential to become Melo, Pierce or even Hayward good, then the last thing I am calling that guy is "low potential".
Jackson on the other hand lacks size and strength for the PF spot, lacks the shooting touch or dribbling skills to play guard. He's basically stuck at SF - and his offensive game needs a HELL of a lot of work. You'd probably need to spend years just developing him into a non-liability on offence. And if / when he looses even a hint of that athleticism his potential drops off like a rock.
So lack of upsidie? Hmm, I don't see it.