I think people like to pull numbers out of thin air sometimes to prove a point. More realistic is probably 6-7%. and that's a good return too. 10% over a long period was possible probably during the mid-late 80's but in recent years, not even close.
And a MILLION would get taxed ~ 1/3, but then you can live "cheaply" and retire off that. But then again, you wouldn't be wealthy because you'd live cheaply! And you wouldn't be wealthy because you'd probably just rent a cheap apt, take the bus, eat ramen at home, and watch non-cable tv.
So it'd be great being a millionaire but have no assests. But hey, you could retire early!
And these days, you'd probably want to have at least $1-2 million LIQUID (after paying off mortgage, loans, cars, etc) to comfortably retire at any age, and enjoy it!