One reason you have to be careful projecting players is the difference between a 10 WAR and 30 WAR career may be some "tweaks," and not a drastic makeover.
Moniak's performance this year is a perfect example, just by becoming more selective in June his OPS jumps 100 points, his BA, SA, K rate stayed the same. We've seen this with Kingery, he didn't drastically make over his swing, he just stopped swinging at too many pitches outside the K-zone.
Now as a player gets older, there are less options for "tweaks," at some point most players are who they are, but as Grienke and Kershaw show, even 30 year old pitchers can adjust to a significant loss of velocity by sharpening their off speed stuff and getting smarter about pitching.
Point is you have to look less at gross performance, and look at the underlying skills, if a hitter lacks bat speed, doesn't matter how disciplined they are, at some level they'll get exposed. Or if a hitter can't recognize a breaking ball. Or a pitcher who doesn't have minimum velocity.
Some skills can be taught, you can sharpen a pitcher's mechanics, shorten a hitter's stroke, but if they don't have body control (repeatable mechanics) or hand to eye coordination and quick twitch muscles, all the coaching in the world won't help.
If you're not developing players, that's a coaching issue, if you're not drafting/signing players with the raw skills to be coached, that's a scouting issue.