*legislate seat belt laws*

*car crash deaths go down*

"See? We didn't need those seat belt laws."

Two qualifiers:

1. Seat belt laws might not be the only thing that decreased them but you can't assume it would have happened anyway.

2. Taking precautions inherently means the thing you're preparing for might not happen. Doesn't make the precaution unnecessary in hindsight.

"I wore a seat belt but didn't get into a car crash. Might as well not have worn it" is an argument often made against gun ownership. Nearly 100% of gun owners never discharge it in self-defense. Does that mean they shouldn't have one *just in case*?