The issue isn't whether you can or can't get yourself to become really good at something if you only spend enough time...the question is how much time? And is that time the same amount of time for all of us? If not, why not? ...could some of those reasons be due to nature?
Why does it take one person 23,000 hours of directed practice to become a chess master while another achieves it in 3,000? While others never ever get there within their lifetime? Is it simply their practice methods? or are there other factors at work?
Why was Greg Norman able to break 90 after a month of playing golf? While others will never achieve that level of skill?
I can recall growing up (as I'm sure we all can) there was always that one kid who could run faster than everyone, could kick a ball farther, and could beat the crap out of most of us. Was this due to his pre-K kickball/boxing lessons?
Finally, if you and I both spend the same of time practicing diligently, will we be equally skilled? If not, why not? can we look in our genes to find an explanation? perhaps not the only explanation, but an explanation.
Simply put, Nature matters! and so does Nurture! Trying to deny the impact of either is the height of ignorance.
Thing is science and psychology are answering these questions. I feel that those who say it's just god-given talent and that most people just can't get there are about the same as people who thought that performing lobotomies were the way to cure mental illness.
People can be neurologically damaged, they can be chemically imbalanced and those things can be sometimes regulated with drugs....and sometimes they can be regulated with willpower as some psychiatrists have found out.
Thus your questions are valid yet they are questions that some like San Jose Dick refuse to even ask. For guys like him every human has a line that they cannot cross no matter how much they try and until I see actual science that PROVES conclusively that an otherwise able-bodied person of sound mind can't reach world class performance with deep practice and training, I will be of the opposite opinion. And that opinion is that every such person can always get better than they are.
And someday, probably in our lifetime for those of us in our 40s, we will probably see it come to pass that we can be tweaked genetically and biologically to BE more adept at whatever we desire to excel at.
we already know that some drugs will do that, adderall, cocaine, ritalin, viagra, pain pills, etc....We know that cyclists have figured out how to use blood transfusions and hormones to enhance their performance.. and steroids, etc...