The price of goods, services, salaries, etc. have all gone up by roughly 3–4x in the US (with some notable exceptions like housing in certain areas and college tuition). Someone who invested in the S&P 500 and has 29x as many nominal dollars as they did in 1976 can buy about 8 times as much of other people’s labor as they could before.
Some fixed percentage of the total size of the S&P 500 is a practically useless measuring stick. If we use that as a standard, 90%+ of the population in the USA is 8x poorer than they were in 1976.