"Isn't the question whether the very best predictor of whether any son(player) is going to be the best they can be based on whether that happens in HS or from ages 18-21, for most players?"
I don't think being a multisport athlete in HS is a predictor of anything, its more like a lagging indicator of athletic talent. These kids are multisport athletes because coaches recognize their exceptional talent and there is a demand for the kids services. Not all talented kids will play multiple sports for a variety of reasons, but I suspect its not because they didn't have the opportunity.
Each level has a narrow funnel, the more athletic the player, the better opportunity he will have to advance, whether he's 14-18 or 18-21. I also think for most players 18-21 is all about seeing where their exceptional potential can take them, when they apply their talent fulltime to baseball. Some flatline, some lose interest, some catch fire, some were misjudged, some get bigger, but only a very few move through the next funnel...... most of the position players on my son's minor league teams were multisport athletes in HS, one of the pitchers even had a football scholarship to LSU.