What I'm looking at, is seeing him play 20 mpg, 26 mpg, 30 mpg, and his per 36 numbers have stayed relatively flat.
What I would expect to see happening is as he's becoming a better player for there to be some variance in his per 36 numbers. He's scoring at the same rate, rebounding at the same rate, basically looking like the same player he's been. I don't say that he's a bad player, but I think his development has plateaued and he is what he is.
I'm not overpaying on the idea that he some untapped potential. The only place I can really find any movement is in his ability to create for himself. And part of me argues that was based more on Cleveland's shitty offense than him becoming a low post threat.
While I see your point, I think that the ability to even sustain his per 36 numbers as his playing time increases is improvement in and of itself.
Jarrett Allen (pts reb / ast / stl / blk / fg% per 36 minutes over the first four years of his career):
14.7 / 9.7 / 1.2 / 0.7 / 2.2 / .589 on 20.0 mpg
15.0 / 11.5 / 1.9 / 0.7 / 2.1 / .590 on 26.2 mpg
15.1 / 13.0 / 2.1 / 0.8 / 1.8 / .649 on 26.5 mpg
15.6 / 12.2 / 2.0 / 0.6 / 1.7 / .618 on 29.6 mpg (w/ BRK: 15.1 / 14.1 / 2.3 / 0.8 / 2.1 / .677 on 26.7 mpg, w/ CLE: 15.7 / 11.8 / 2.0 / 0.6 / 1.7 / .609 on 30.3 mpg)
Compare these to the per 36 numbers over the first four years of the top seven guys on the list of highest center salaries posted by LeeBuckeye a few pages back and draw your own conclusions on how much a young player's per 36 numbers should be improving over the first four years in the league.
Gobert:
8.6 / 12.9 / 0.6 / 0.7 / 3.4 / .486 on 9.6 mpg
11.4 / 12.9 / 1.8 / 1.1 / 3.2 / .604 on 26.3 mpg
10.4 / 12.4 / 1.7 / 0.8 / 2.5 / .559 on 31.7 mpg
14.9 / 13.6 / 1.3 / 0.6 / 2.8 / .661 on 33.9 mpg
KAT:
20.6 / 16.5 / 3.1 / 1.1 / 2.6 / .542 on 32.0 mpg
24.5 / 16.9 / 3.7 / 0.9 / 1.7 / .542 on 37.0 mpg
21.5 / 17.3 / 3.4 / 1.1 / 2.0 / .545 on 35.6 mpg
26.6 / 18.0 / 4.9 / 1.3 / 2.4 / .518 on 33.1 mpg
Embiid:
28.7 / 11.1 / 3.0 / 1.2 / 3.5 / .466 on 25.4 mpg
27.2 / 13.0 / 3.7 / 0.8 / 2.1 / .483 on 30.3 mpg
29.4 / 14.6 / 3.9 / 0.8 / 2.0 / .484 on 33.7 mpg
28.0 / 14.2 / 3.6 / 1.1 / 1.6 / .477 on 29.5 mpg
Jokic:
16.5 / 11.6 / 3.9 / 1.6 / 1.0 / .512 on 21.7 mpg
21.6 / 12.7 / 6.3 / 1.1 / 1.0 / .578 on 27.9 mpg
20.4 / 11.8 / 6.7 / 1.3 / 0.9 / .499 on 32.6 mpg
23.1 / 12.4 / 8.3 / 1.6 / 0.8 / .511 on 31.3 mpg
Vucevic:
12.5 / 10.9 / 1.4 / 0.9 / 1.5 / .450 on 15.9 mpg
14.2 / 12.9 / 2.0 / 0.9 / 1.1 / .519 on 33.2 mpg
16.1 / 12.4 / 2.1 / 1.2 / 0.9 / .507 on 31.8 mpg
20.3 / 11.5 / 2.1 / 0.8 / 0.8 / .523 on 34.2 mpg
Turner:
16.3 / 8.7 / 1.1 / 0.7 / 2.3 / .498 on 22.8 mpg
16.6 / 8.3 / 1.5 / 1.0 / 2.4 / .511 on 31.4 mpg
16.2 / 8.2 / 1.7 / 0.7 / 2.3 / .479 on 28.2 mpg
16.7 / 9.0 / 2.0 / 1.0 / 3.4 / .487 on 28.6 mpg
Capela:
12.8 / 14.4 / 0.8 / 0.4 / 3.6 / .483 on 7.5 mpg
13.3 / 12.1 / 1.2 / 1.4 / 2.3 / .582 on 19.1 mpg
19.0 / 12.2 / 1.5 / 0.8 / 1.8 / .643 on 23.9 mpg
18.2 / 14.2 / 1.2 / 1.0 / 2.4 / .652 on 27.5 mpg
Not trying to be argumentative, just wanted to post these and see if you still see things the same way. I can see rational arguments to be made on both sides, but hopefully these numbers, which I tried not to cherry-pick to fit one side or the other, can help everyone form more meaningful opinions.