10,000 hours is a myth based on a study of violin players at a school for performing arts. Researchers found that a correlation between grades and how much students practiced (shocking). Most "A" students estimated that they had practiced over 10,000 hours in their life, while students below that GPA had practiced less than that.
If that one study is valid (and that's a big if because it hasn't been replicated) it means that 10,000 hours is how much you need to practice to get an A in music school, not learn an instrument.
It's very easy to practice ineffectively and waste a lot of those hours without getting better.
10,000 hours is a lie.