I think you're confusing 'due process of law' with some sort of conspiracy. We convict people or not based on evidence and the fact is physical evidence degrades over time. This is true of pretty much an crime. Somebody charged today with a crime that happened 30plus years ago isn't likely to get convicted regardless, that's just the way things are.
The point here I suspect is there is currently great public pressure to prosecute people due to the publicity surrounded operation yewtree and the like. It looks like the CPS is bowing to that pressure a little and prosecuting cases which, out of the public eye, they would probably abandon. Whether they are actually guilty and the damage their reputation may suffer from being prosecuted are less of a consideration.