You have to take into the account that the S&P 500 is, by definition, an index. Stocks can move on and off the index depending on how well they are doing. As such a headline index such as this can ONLY move up in the long term. If it were to move down in the medium and long term what you would be looking at then would be beyond the idea of a collapse in an economy but the collapse of a civilization.

As such indexes make more reliable investments than individual stocks and is the reason that many individuals that run funds sneakily invest their personal funds in trackers.