Someone - it may be the journalist writing your linked article, or the professor he quotes, or you - is confused about the difference between the lifespan of a company, and how long it stays in the S&P 500. Observe:

The average lifespan of a company listed in the S&P 500 index of leading US companies has decreased by more than 50 years in the last century, from 67 years in the 1920s to just 15 years today, according to Professor Richard Foster from Yale University.

Professor Foster estimates that by 2020, more than three-quarters of the S&P 500 will be companies that we have not heard of yet.

Very well, but that does not demonstrate that the existing 500 will be bankrupt, only that they will be smaller than, say, Public Service Enterprise Inc.

Additionally, the good professor seems to have a definition of lifespan which is spectacularly non-useful for your purposes:

"When a company has run its course it gets bought," says Professor Foster.

You are interested in bankruptcies in which the company ceases operations. Buyouts, in which there is new management or even the same management but new ownership, are irrelevant. By that standard, YouTube is out of business!