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And if the S&P drops like it did in 1987, 2000, 2008, then what...?

Then just wait for a few years and it will recover again. https://www.google.com/finance?q=INDEXSP:.INX

Who will pay the bills in the meantime?

Just run your projections through http://www.firecalc.com/index.php and project what your historical odds of going bust are.