juiio, a good planner would explain why your scenario is not a safe assumption. Since the percentage returns are only averages, but the amount of money most people would want to take out yearly is fixed, in many cases you can eat up your money faster than would be expected.
You are right that over the long term, it might average out that way, but when the investments have an off-year or two (and it's going to happen!), the value of the investments could actually drop 15% in a year. If the investor still wants to take another 7.5% out every year - even in the down years - it makes it that much harder to catch back up. As long as the investor is willing to forego withdrawals in the down years, things are fine. But most people, once they start drawing that money every year, aren't willing or able to give it up.