As lovegtm astutely mentions above, the ability to generate unassisted threes can set up the rest of Tatum's offense, where in Pierce's early career, his strength that was the cornerstone of his driving ability was key. I would be great for him to have a crazy quick first step, but if he did, maybe he would have never developed the rest of his game, like so many ath-a-letes do when they dominate HS and college play by physical attributes alone.
Tatum reminds me of a 1970s forward with great scoring skills. My first comp for him was George Gervin (with a bit less flavor). I see him as a perennial 25 ppg scorer. It makes sense that they're retooling his game to eschew the long two, but I do like the idea of him exploiting smaller switches at the FT line, a la KD, and I like his mid-post turnaround in small doses. One of the reasons he's had trouble finishing at the rim this season might be because he's starting his drives past the three point line, when in the previous two years, he wasn't always starting from there. He is adjusting, as the recent at rim FG% attests. His eFG% is down, but he has bumped his scoring from 15 to 20 ppg, and also upticked his rebounds, assists, FTA, and steals.
I'd say he's showing solid growth so far in year three on offense, and has really improved on defense. He might not become a top 5 player in the league, but probably will be top 15.