Do you have any evidence that your suggestion actually works for real people? All studies on this I've come across say that anyone using common wisdom will both exit and re-enter too late -- i.e. they tend to end up selling low and buying high, and even just holding would be an improvement.

(I'm saying "even just holding" because a constant-fraction rebalanced portfolio is a disciplined way of actually buying low and selling high, and not only in a bear market.