First off, what is your source for that average? This report (https://www.innosight.com/insight/creative-destruction/) has a much higher average, and forecasts a fall to 12 years by 2027.

Second, that's average time in the S&P 500 index. As noted and n the report, there are many reasons besides going under that companies drop off the index. That number doesn't really say much about longevity of the company.