heh
There is a practical consideration for a statute of limitations. Evidence in crimes cannot be kept indefinitely until a suspect is found. Cases grow cold over time. Degradation of the evidence cannot always be preserved. Do you expect all evidence to be stored forever? A stolen watch retrieved from a pawnshop is kept in storage until the thief is found? I think the owners
would object after a few years. Besides, try explaining where you were ten years ago when someone matching your description was seen wearing a watch like that? Oh, you owned a watch like that? Prove it? Oh, so you sold it after the battery died? Prove it! So the pawn shop closed after the pawn shop owner died? Tough! A statute of limitations closes unsolved cases off the books, permanently. But certain crimes are considered so abhorent that they have no limit such as murder or kidnapping.
How? If strong evidence comes up later in life against the accused, how would that go against the Sixth Amendment (US Amendment?).
In my more cynical moments, I would imagine that this also has something to do with reducing the workload that our police force is expected to handle. You cannot realistically expect the police to be constantly poring over every unsolved offense that ever took place in their jurisdiction. Manpower (humanpower?) is limited, and it only makes sense to focus it on the cases which the police force will be most likely to be able to solve and which demand attention: that is, the most recent offenses. Their ability to do so would be compromised if they were expected to be actively solving the question of that rash of shopliftings in the mall in 1967. Hence, statutes of limitations.
But if overwhelming evidence does come up later, I fully expect the police and prosecution to look into it a bit further. Why should a sexual offender get off free because of a time limit?