1. Look at the Erhard Wolf book on Jaeger rifles. A few of the later rifles show more pronounced heals. Yes... they're still pretty flat... but it shows a beginning of the evolution to curved butt plates. These correspond to similar styles used in America at the same time (roughly 1770's).
2. The American rifle tradition evolved with German immigrants.
3. Because American gunsmiths were less restrained by more formal styles, I believe they were more free to change and evolve... for better or for worse... and there doesn't need to be a logical, or scientific, or engineering, reason for it.
4. "Calibers got smaller because all the big game was gone". I don't believe that myth. I think it was partly due to economics. Small gun... less powder and lead to buy... less expensive. What I've read leads me to believe the smaller calibers were starting to be used it poorer areas first and moved out from there.
5. You're speaking too much in absolutes... "clearly"... not really... "you have to agree"... no I don't.
6. "More art than science"... you're looking too hard at cause and effect. Not everything is done for logical reasons. It reminds me of a line from a TV show... "it doesn't always have to make sense... like plus size bikinis". Barrel length and weight may have nothing to do with the shape of the butt plate.
7. I've read far more period journal references to shooting from a rest than I have about standing square or angled in relation to the target.
8. Some competitive shooting matches may have established positions that you were expected to use. I believe these evolved from formal matches in Germany. Back county matches probably didn't. They could shoot however they wanted.
9. There are period paintings of shooting matches. A good one is "Shooting for the Beef" by George Caleb Bingham (ca. 1850). It shows a back country shooting match.
10. Crescent buttplates may be bad for some shooting positions... but some shooting positions are just plain bad in themselves. Remember all those drawings of match shooters from the 19th century in contorted positions? In hindsight they don't make much sense either... but they did it.
What does this tell you about the shape of a buttplate?
Sometimes we just have to accept what "is"... or in this case "was". There doesn't need to be an explanation. The same holds true for ourselves. Lets accept ourselves for who we are and accept the longrifle for what it is.