You're spot-on,
@OldCop. Your question, "[H]ow long is too long?" is
the question when it comes to long-unsolved cases.
In your post, you explained all the valid reasons for the policy of holding back a case file. What I propose, though, is that investigators
should use the power inherent in crowdsourcing on the internet if an arrest is not made within a certain period of time. This way, investigators can obtain new leads & harness knowledge that they do not have.
For example, the murder of Betsy Aardsma at Penn State occurred 50 years ago. There hasn't been an arrest in the case. The Commonwealth's "Right-to-Know Law" exempts records of a criminal investigation from disclosure. An amendment to the statue could remove the exemption -- say, after 50 years -- so that the records could be disclosed. Once disclosed, thousands -- possibly hundreds of thousands -- of individuals would be able to pool their brainpower to "work" on the case.
Admittedly, investigators & prosecutors may be reluctant to make such records public. They may fear damaging a case or disclosing facts that damage the reputation of an uninvolved person. This is why I favor a
legislative solution to the issue: state legislators can decide the right "balance" for their state or commonwealth between the need to protect the evidence and the ridiculousness of continuing to keep case records sealed even after all participants may be deceased.