By whom?
Yeahbut! Gladwell didn't say it just takes 10,000 hours to succeed. He said it's one of four essential ingredients. The other three were being early, having the talent, and doing something that is or will be in demand.
10,000 hours: Gladwell is careful with his math. The Beatles played three sets each night for German audiences — about eight hours in all. Just four years of that gets you past the 10,000 mark.
Being early: Along with Dylan, they radically broke away from the rockabilly/doo-wop/soul mold — after those formative hours and years of playing old-school rock and learning to play it well.
Talent: They were gifted musicians, right?
Demand: Young people were getting bored with the same old rock 'n' roll. Some critics had already declared rock dead. Teens needed something new. So when they heard "I Wanna Hold Your Hand" and "I Saw Her Standing There" come out of their radios, they were hooked.
Gladwell demonstrates the same thing in the high-tech industry, pro hockey, rice-growing in China, and more. Most readers and critics found it persuasive. If you read the whole book, I'll bet you'll be persuaded, too:
Outliers: The Story of Success