Crimes need investigation
Given the detailed enquiries which have failed to come up with evidence of conspiracy or improper conduct of the kind alleged by climate denialists, and the consequent controversy over UEA's quality of research work, having a proper criminal investigation of the break in seemed to be appropriate, so long as there was a probability of discovering who was behind the criminal activity.
I don't think criminality is subject to a 3 year statute of limitations. As I understand it, that law concerns civil damages, and criminals can be tried many years after an offence was committed when and if new evidence comes to light. If the break in was paid for by lobbyists working for fossil fuel producers or other parties with a vested interest in discrediting UEA's line of research, another investigation in future might be to follow the money.