First, you can not buy shares in SPX as it is an index that has no shares available to buy or sell.
Second, Buying an index and holding is not the sure thing that it is assumed to be based on this long running Bull Market. Check it out --- From 2000 to 2010 the S&P 500 LOST money. Not to mention 1929 to about 1945
Per your logic, last Friday market didn't go up.
Because it was sometimes on the day going down for a short while.