The caveat is that you will not be the best. You might not even be world class. Or even national class. But you are going to be better than somebody who practiced only 1,000 hours no matter how talented they are.
What people forget is that at the very high levels of achievement in any field, 20,000 hours is just the price of entry. Everyone who's playing has practiced for 20,000 hours. Difference between 1st, 2nd, and 10th comes down to talent and a bit of luck.