So the error there is that purchases are not actually independent. If say dot com stocks have been growing ten times as fast as the rest of the stock market, they can't do that forever (eventually they will become the whole stock market, then the whole economy). If a government keeps offering higher and higher real interest on bonds, eventually it will default or trigger high inflation. So the wise investor buys lots of different things, knowing that today's darling will be tomorrow's ugly sister. I recommend a good textbook.