I used to swim a lot, and was doing so when Michael Phelps got his first big haul of olympic medals. I remember that at the time there were people who virtually accused him of cheating because he had incredibly big feet which worked like flippers and gave him an "unfair" advantage. Of course nobody really thought it was cheating, but it did give an advantage, and all other things being equal, no amount of training would ever result in growing bigger feet.
I mention the swimming thing just because it's simple and clear cut - I've just looked up Michael Phelps and he apparently has size 14 feet plus an arm span of 2 metres and it's easy to measure these things, if less easy to quantify their exact benefit.
When it comes to someone like Mozart, nothing can be measured, yet most people would agree that he had some sort of very special talent that no amount of study or training would ever bring most people anywhere near.
In a rather roundabout way, that brings me to my point: I think you have to have real talent to excel on an instrument (I will never be a Bosgraaf or a Petri no matter how much I might try). Some people seem to be fortunate enough that they can excel on more than one instrument, but maybe that is simply a rarer talent than being able to excel on just the one.