I think the amended version of '10,000 hours' is closer to being correct, and probably not the one the public wants to hear.
Ericsson's research isn't much better either though. His sample size was very small and his research was never replicated. As the 'how I taught myself physics in one year' thread showed, it's pretty obvious innate talent exists and can allow people to attain mastery of very complicated concepts in far less than 10,000 hours.
Not sure it is a cruel myth...more like wishful thinking that can have unintended consequences by mismanaging resources (such as having low-IQ kids in prestigious schools, in the hope that environment will overcome a cognitive deficit)...if it were so cruel, it would not have taken the world by storm...'10,000 hours' succeeds as a meme because it tells people what they want to believe, that with enough practice, anyone can covet the skills of genius. It's not so much that people want to become world-class musicians or top physicists, but rather that they have the potential to become those things if they want to, by practicing enough.