Again, from the outset, I said it was NOT a comparison. I was just giving ONE recent example of a guy who changed his accuracy and completion percentage with coaching and hard work.
And let's not pretend that Stefon Diggs is the sole reason for this reversal of a Allen's accuracy stat. Your statistical analysis would have to include some sort of conclusion that the majority of Diggs' receptions were uncatchable ducks that Diggs managed to salvage, and I don't think that is the case.
And, look, I'm not mad and I'm not going to get overly critical, but the way that you sum up Allen's numbers in Buffalo is a bit misleading. Allen was not "in the 54-56% range" for two consecutive years. In Year 1, he was at 52.8% and in Year 2 he was at 58.8%. The reason why this distinction is important is that Allen did NOT have comparable accuracy rates in both Years 1 and 2, he made significant improvement from Year 1 to Year 2. And then improved even more in Year 3.
The elements I'm focusing on have nothing to do with who the WR is. You can't guarantee who your WRs will be for your entire career. Improving accuracy has to do with coaching, mechanics, and the desire to change and work hard. Allen only started for 2 years at Wyoming, and Wyoming is not really considered a hotbed of QB coaching. I remember reading an article (which was primarily about another QB in that draft) which talked about Allen getting additional coaching to improve himself for the draft. And now that he has quality coaching, he has improved each year, which is often a difficult thing to get an older player to do (i.e., older than a 17 year old recruit).
Again, NOT a comparison to Josh Allen, merely a rebuttal to the general concept that you can never improve a QB's accuracy and that a HS completion percentage will extrapolate across an entire career.