yes, that is the argument Jack Bogle used, I get your point, but you still only
own about 50% of the global stock market if you only invest in the S&P 500. For those that like indexing strategies, I think you have to make your equity allocation equal to the global equity allocation if you want to participate in the global markets proportionately. I think over the next 10 years the international markets outperform the US markets like in 2000-2010.
I am not advocating getting out of US stocks, I am simply saying make your stock allocation equal the stock distribution globally. This calls for about 50-50 US vs. International. Most people aren't doing this and they will just follow the herd. By the time they get more international exposure, prices will have increased and they will be selling the S&P 500 when US stocks are down and buying International after having increased substantially.