If you invested $1,000 in an S&P 500 index fund at the end of 2004, it would have been worth about $745 at the end of 2008. But you only would have lost $255 on the investment if you actually sold your shares at the end of 2008. If you left your money invested, you would have recovered back to your initial $1,000 investment sometime in 2010 ... and by the end of 2014 your $1,000 investment would have been worth almost $1,700.
And even after the massive sell-off over the last week, your investment would still be worth more than $1,900 right NOW.
You are assuming that all crashes will bounce back at the same rate. Since I think the last recovery was really a bubble, at some point there will not be a bounce back. If you look at the stock market after the depression, it was slow going.