I don't think that the development of Kei'Trel Clark, Jon Gaines, and Owen Pappoe (to name few) are really going to impact the team's win/loss record.
Also, how much marginal improvement do you expect from PJJ, Garrett Williams, Dante Stills, and Michael Wilson? Those guys are already major contributors, and you've asserted that PJJ and Garrett Williams are already among the elite at their positions.
The case for big Cards improvement from young players is that Beej comes back much stronger from the last time we saw him (which was him getting zero sacks and three pressures over the last six games of his rookie season), Darius Robinson shows why he was a first-round pick, and Marvin Harrison Junior matures from WR26 in fantasy (using this as a rough heuristic for positional rating since it covers three dimensions) to like WR12.
You are basicly making my point with this.
Of the 21 players some will not pan out. And I 100% agree that Clark, Gaines, Pappoe and WR Palmer are the most likely candidates. QB Tune is a huge ?
BJO and DR were high draft picks that contributed nothing due to injury. I absolutely expect an impact from both - especially when the baseline is 0. Add CB Jones to that list.
9 of 10 Elite OTs do not reach their level until their 3rd year. I expect MHJ to develop. For both, we do not know if that means all-pro level. But I certainly expect PJJ to move up from his 12th ranking and MHJ from his 21st ranking.
You are probably right that Stills will not develop further
That leaves 10 additional players + Higgins, CB Thomas and WR Xavier. Yeah, I absolutely expect to see a leap forward from some of those 13 players
Melton showed he belongs, but had great games and terrible games. I expect more consistency and a higher floor.
Reiman and Adams showed elite capability in one aspect of the game, but struggled in the other aspect. I expect Reiman to be more involved in the passing game and Adams to develop in pass-pro.
I expect DTD to take over and be an improvement from Thompson's play last year.
I expect Higgins to mature as he has not played TE that long.
Christian Jones showed improvement from Pre-season to end of the season when he was forced into action. I expect that to continue.