I take it that you are saying they would be materially better if they had specialized earlier. Just like discussing the opposite, it is speculation. If they had played earlier, perhaps they would have burned out, they would have been injured, they would have struggled so much or put so much pressure on themselves to be good that they would have moved on to another sport (since both are talented athletes).
At any event - GDA, ECNL, ODP, PDP (up here in NorCal) - we see talented players who play a very narrow (often it is very direct, taking advantage of speed or physical size). So many coaches want to coach that talented player and believe that they (the coaches) can be the ones to teach them tactics, discipline, the finer skills. And some of those kids NEVER learn. You will see them at 14 and 16 and they are the same kids they were at 10 or 12. Of course, we'd rarely see a kid like that on the national team unless that one characteristic - say BLAZING speed or incredible strength - will be limiting as the player gets older if she does not develop a more well-rounded game. Maybe moving to competitive at the advanced age of 14 actually allowed Morgan to develop other athletic - perhaps non-soccer - skills that have allowed her to succeed as a goal-scorer. Sure we can pick apart her game but that strikes me as silly - she's on the best goal-scorers ever (no debate on that). If someone said that your child would be one of the game's greats, beloved by fans, dozens or hundreds of caps, multiple goals in a world cup but she would not be a well-rounded player . . . would you take it? Put another way: aren't we sort of quibbling when we think about how much "better" Morgan or Mace MIGHT have been since we are really talking about marginal possibilities that these supremely talented players would be better than they are now?
I didn't choose the examples. Those examples are the best argument for one side of the debate. But even for those examples, the relevant question is how much better they would be and whether the extra effort is really worth it. I'm sure Mace would second guess not focusing earlier if she doesn't make a world cup team. She'll have to ask herself whether HS volleyball was worth it. The margins are very small.
And if you are really arguing that playing less soccer is going to make you a better soccer player, I don't know what to say. I guess I could argue that that allowing my kid to play more video games instead of studying will make him a better student. It is possible, right? Under some theory? Of course, there are plenty of kids that went to Harvard that spent too much time playing video games in high school. But I wouldn't use them as an example of good study habits for my kids.