If the margins were tiny in reality rather than on paper or based on Bundesliga form it would be a more compelling argument. Or if he put in 90 minute performances occasionally to suggest he is close, that would be akin to your sprinter eeking out the milliseconds, we'd only need to eek out consistency across game time. But compared to top wide forwards in the league the margins do not appear to be tiny.
The problem with Jadon is it's not just one thing, it's many things that are lacking. Which is why I think the manager took that drastic step. He realised everything so far isn't working and there's a lot to work on, so doing things conventionally wasn't the option. I don't believe he would do that for a few technical disciplines, or even a single physical or mental component that he could work on himself. So even this suggests to me that the manager believed it wasn't about margins.
My biggest issue is it's taken him this long to even meet the basic criteria of being a physically robust, Man Utd player. If your sprinter wasn't fit enough to run the distance, I think you'd question his mentality and approach to his discipline and on that point it isn't about margins it's about the basics. That's Jadon, he has a habit of getting subbed, and he rarely produces across a full game as evidence of this flaw. Maybe he worked on this in his camp, but it's been stop start since then and hard to judge. I think this acceptance of his own mediocre physical standards is probably related to the passivity he shows on the pitch, it seems to be part of his character that he has to be pushed and cajoled.
Then we get into the footballing side and that's your sprinter having to work on his gait, his rhythm, whatever they work on - I've no idea about sprinting. On this point I think most of us have a desperation about wanting him to do well, because we all like to see an aesthetically pleasing, technical player. Kagawa and Mkhitaryan got the same treatment. We looked for it and looked for it until it was no longer tenable to support the idea they'd improve. I think Sancho is approaching that crossover point. At this point it's probably 50/50 as to whether he goes the same way as them, and I'd say he'll probably get next season to display which it will be.