What is the point of this thread?
Person A gets murdered. It's investigated (and for once Garry is right, everything is thrown at it), but like all crimes sometimes there simply isn't the evidence to convict and no matter how much you throw at it, there will never will be. In some others, depending on what forensic evidence could have been gathered at the time, there may be a chance in the future. But not all evidence that might be usable in the future can be preserved. For example, DNA evidence didn't exist 100 years ago, no one could have foreseen it, so preserving evidence that could or might contain that wouldn't happen.
All of that is common sense. As to be honest is the answer to the question.
I was watching a program on an cold case,this one being 24years old the local police had investigated,it had been investigated as an cold case file with no luck,then out of the blue its was picked up by an noise abatement officer who look into it in his own time,and he cracked it in 2weeks,it was just a little something both investigation had overlook,a name on the list that nobody had bother to interview.
So their always a chance.