In short: becoming world-class is getting to the top N percent and depends on the number of people willing to practice for being world-class.
It seems that currently we've settled to 10 000 hours because 10 000 hours seems to reduce the number of candidates enough to make each "survivor" a real expert.
If suddenly more people started to practice 10 000 hours, you would possibly need 15 000 hours to distinguish yourself back to the top N percent among the horde of damn good (by our current standards) piano players, programmers, or whatever.
Being world-class also depends on the subject: the world can take only so many world-class piano players because the audience is somewhat limited, but it can absorb many more programmers because there are roughly infinite number of interesting and useful project to be done.
It seems that currently we've settled to 10 000 hours because 10 000 hours seems to reduce the number of candidates enough to make each "survivor" a real expert.
If suddenly more people started to practice 10 000 hours, you would possibly need 15 000 hours to distinguish yourself back to the top N percent among the horde of damn good (by our current standards) piano players, programmers, or whatever.
Being world-class also depends on the subject: the world can take only so many world-class piano players because the audience is somewhat limited, but it can absorb many more programmers because there are roughly infinite number of interesting and useful project to be done.