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I think the following is the heart of the argument.
We were all so comfortable with the conventional wisdom of investing for retirement — "buy and hold," "save for the long term" — that nobody bothered to note a couple of things that are obvious in retrospect. First, saving for college isn't like saving for retirement. The run-up is much shorter, 18 years at most instead of 30 or 40, so most of the miraculous gains of compound interest are lost. Second, the payout is far more immediate and inflexible. People can choose when to retire, delay if they have to, and ride out ups and down in the market over decades. For most students, college happens three months after graduation, ready or not, and the check is due on Day 1.