When every investor thinks a company's stock does well,...
When every investor thinks a company's stock does well, its price gets bid up and reduces future returns. Take a look at how well Union Pacific railroad did historically compared to the S&P 500. It's a boomer stock that most investors don't care about, but its stock behaved like a big tech stock. Overweighting 2T market cap stocks that have had incredible returns is going to be a good strategy, but not a great strategy for the long term unless you believe the companies that are winning today are the same as those winning in 20 years.
Moreover, keep in mind that most index funds are already quite heavy in big tech, so if you own $SPDR S&P 500 ETF(SPY.US)$ or something, you already have heavy exposure to these companies.
I'm not saying your portfolio needs to be full of companies like John Deere, American Water Company, etc. Certainly $American Water Works(AWK.US)$ is not going to outperform $Alphabet-A(GOOGL.US)$ (probably). $Invesco QQQ Trust(QQQ.US)$$Apple(AAPL.US)$$Microsoft(MSFT.US)$$Amazon(AMZN.US)$$Netflix(NFLX.US)$
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