My wife and I call this the "faith in experts" argument. We named it because we often find ourselves on opposite sides of it. She believes in experts, I'm more skeptical.
The main thing I've realized is that returns to pure capital just aren't that high. For a typically-sized retirement account (few hundred K), you're much better off going hard on your career if you're in a field where that's rewarded (law, finance, tech, med, etc.)
The one saving grace of your idea is that you'll have made all the mistakes by the time you have real money to manage, so you won't make silly dumb beginner mistakes like buying high and selling low. That's my post hoc justification, anyway ;)
