Means Motive Opportunity
"Home Office that retaining the DNA of people who were innocent at the time of arrest had helped to solve crimes they committed years later."
The DNA isn't just a tool for investigation of crime, it's also often the primary evidence! If you take DNA samples of everyone, then it becomes the first line of filtering and so by definition of the filtered set you've chosen, the DNA will always be a deciding factor in the arrest of criminals and the primary evidence of 'opportunity'.
So it's a weighted set.
And really you need to know how much it's also the causality. i.e. if you arrest fingerprint and DNA sample innocent people, how many of those people feel they have less to lose if they commit a crime? They have a record already, it will follow them around already, and negatively impact them all through their lives, so what difference does it make if it's a 'criminal' record rather than a 'suspected criminal' record?
Then you have the innocent until proven guilty bias. If someone is arrested for murder and then released, and you know this, then the next murder you are more likely to assume they did it.
That is why a jury is *not* allowed to see a defendants *non* convictions. Because it skews their perception of the evidence presented to bias that information against the defendant and results in a biased set. But that protection doesn't extend to investigation, where evidence of non convictions are used during investigations.
But most of all, I don't want it. It's as simple as that, I don't know what horrors DNA cataloging everyone will lead to, but then I don't know how much information can be extracted from those DNA records yet, and nor do you, DNA cataloging is in it's infancy and UK plc should not be the test bed for it. Leave that to dictorships to test, it's the sort of thing they're good at.