Confused malcontents swilling Chardonnay while awaiting the Zombie Apocalypse.
I guess it's possible that they've IDed the perp, but he's dead and the evidence wouldn't convict in a law court.
To go back to the 60s/70s and convict, you really need DNA evidence or a confession.
Police in the UK do retain evidence in unsolved cases, for ever.
The oldest case solved by DNA was a 1956 murder case in Montana. Unfortunately the man IDed had died, but the family voluntarily took DNA tests which proved their father was the killer.