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Long-time investor, beginner question

I just started learning about investing at the age of 33, so bear with me…

Let's say I have $10,000 to invest and I want to invest all of that money in Apple Inc. (AAPL).

Just for the sake of argument, looking at Apple's ROE for as far as 2008, I realize it almost never went below 30%, and let's say I want to commit to this investment for at least 20 years (meaning I will buy Apple shares worth $10,000 today, and never ever touch'em for at least 20 years!).

So, doing a quick compound interest calculation (using this, for instance); my shares, bought at $10,000 today, can be worth $3,747,379.65 in 20 years (if not more).

I know I am oversimplifying this, but is there an elephant in the room, or am I missing something REALLY BIG?

Thanks.