but as that trend reverses, I expect the share of our economy held by the top 500 corporations (that's essentially what the S&P is) to contract. I agree with t

but as that trend reverses, I expect the share of our economy held by the top 500 corporations (that's essentially what the S&P is) to contract. I agree with this statement, but not with the conclusion you seem to be drawing from it. It is quite likely that the share of the world economy held by the top 500 US based corporations (S&P 500 only includes companies that are actively traded in the US...) will decline. The world economy is becoming more diverse as other nations rise to prominence, wealth and technology slowly and finally start to spread and more and more startups are created even within the US. So, yes the S&P 500 is likely to represent an ever smaller percentage of the total market. However, you seem to be assuming that means the S&P 500 itself will contract. You must remember that the world economy is not a zero sum game. It is quite plausible (I will not speculate on probabily here, but very plausible) that the S&P 500 in particular and the US share of the world economy shrinks in relation to the total, while still growing in an absolute sense. Even if you really believe the actual value of the S&P 500 in particular and the US economy in general will decrease in absolute value (not just relative share), then the answer is not to refuse to invest, but to diversify and include more foreign stocks.