Interesting thought - I dunno if it's true, but it's certainly something to think about. You're saying that the only way to become a successful pro these days is to have a big score very early in your career, right?
I suppose first thing's first, there's no denying a huge tournament cash (and the attention that comes with it) is a great way to kick start your professional
bankroll. It's also about the only way for someone who was previously unknown to get themselves sponsored by a major site, which is obviously a major help to a professional career.
In the end I think it really depends on what your definition of "professional" is and what your goals are - do you want to get on TV and earn sponsorship money, or do you just want to win money at poker and keep your anonymity?
I'd say it's very difficult to get sponsorship without major tournament cashes but it's certainly possible to become a professional by grinding up from the lowest games in anonymity too and you don't even necessarily need to start young to do that.