>Evidence degrades

Maybe in the past, but with modern technology that isn't always true. Statue of limitations comes from a time before even cameras existed.

The purpose of the law isn't revenge.

Also, I see many other people in this thread confusing criminal law and civil law, which is a bit sad to see on a place like HN.

It absolutely is, if you arent going to punish someone who hurt my child then I will do it myself.

The state isn't in the business of revenge. It prosecutes crimes on your behalf because private vengeance escalates and frequently lands on the wrong person.