Yeah, it's the power of compounding and it has been discussed here many times.
I think it's a mistake to 100% count on past performance as your benchmark especially in sector investing which is what I do. Oil for example has a pretty good LT track record but will probably underperform over the next 20 years. Tech on the other hand continues to be more and more of our lives and should continue to outperform.
There were periods in history where the S&P underperformed for over a decade. Now that was a different S&P index than what were have now and its worth noting. The powers that be are constantly tweaking the current index for performance- dumping losers and adding winners. This is what makes the S&P ETF's a very good investment now-a-days including the fact it's a large % tech weighted.
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Wow, that whipsaw in the markets happened fast.