i have had this discussion many times and i believe that with the correct practice that anyone, besides for medical reasons, is able to reach a very very high level of skill. Of course people learn faster than others and not everyone is going to be able to be world class, but I believe the skill level that we are all able to reach is much much much higher than most people think.
i am often met with a response like "I have tried _________ so hard and so long and no matter how hard i practiced or tried I couldn't get good or even decent. I got better but I could never become proficient." The reason I believe many of these people are not improving is because they have poorly developed prerequisite skills and are approaching the activity the wrong way. Almost every activity or skill is not a single skill but is actually the coordination of many skills into one.
the way I think about it is imagine you asked a toddler to juggle. no matter how hard or long a toddler tried, 10,000 or 100,000 hours, they most likely would never be able to juggle. the reason is juggling requires many prerequisite skills:
-need to be able to accurately throw a ball with your left hand
-need to be able to throw a ball with your right hand
-need to be able to catch a ball with your left hand
-need to be able to catch a ball with your right hand
-need the hand-eye coordination to perform these actions simultaneously
i believe that many people who have extreme difficulty at learning something, like the toddler, have poorly developed prerequisite skills that are needed. Some people are unable to simply start juggling, play the piano, or learn algebra. Maybe if they tried breaking down the activity into components of required skills and work on developing those skills they might be able to finally succeed.
the reason I believe so strongly in this is because of myself. I have done a lot of competitive activities and played a few instruments and often have been able to compete nationally in many things. People think I am naturally gifted but that couldn't be farther from the truth. Every activity that I started I was bad in the beginning. Not only was I bad, i was worse than just about anyone I knew who was also starting. Gradually I improved, put in a TON of practice and was able to succeed. Furthermore once you get good at one thing it becomes easier and easier to get good at other things considering many things require the same skills of hand eye coordination, speed, and dexterity.
When I was younger I entered a lot of video game and karate tournaments and also learned to type at about 150 words a minute. During highschool I spent 3-5 hours a day playing guitar for years. I just started playing piano a year ago and my teacher thinks I am some amazing talent but I'm not at all, I just already have developed a lot of the skills necessary to play the piano. I am not trying to brag and say how awesome I am. I am trying to say the opposite and that I am the most ordinary person and I just put in a TON of practice and set out to master many things. I strongly believe that if people approached whatever they were trying to master in the correct way they would most certainly be able to reach an extraordinary level of skill.