Well, you aren't necessarily better sitting out entirely.

When the stock market slumps like that, continual investment lowers your average cost basis.

e.g.:

Year 1: Buy 1 stock at $100

Year 2: Buy 1 stock at $50 (post-crash)

Year 3: Buy 1 stock at $60

Year 4: Buy 1 stock at $70

Year 5: Buy 1 stock at $80

If we just look at "Year 1 = $100, Year 5 = $80" then it looks like we lost $20/share.

However, our average cost basis is $72, so we actually made $8 per share.

That’s not really a great return but it’s not horrible considering the circumstances.