DNA sampling everybody is a very good concept.
Some practical difficulties to overcome. The cost is collecting samples for the whole or a majority of the population is likely to be prohibitive. Not everyone trusts the competency and efficiency of the central agencies. How many times have they interrogated suspects and let them go free to carry on their criminal activities until they are eventually caught many years and many victims later? Not everyone trusts the honesty of the central agencies, if they have a sample of your DNA they could if they wanted to plant the evidence. There are cases where they have abused their position in order to make life easier for themselves, another central database gives them another avenue for such abuse.
I agree with you that it could be a powerful deterrent for criminals but this is a double edged sword. As soon as these criminals notice that the system is faulty or open to loopholes, they will be laughing their heads off and encouraged in thinking that they can get away with it. I am no psychologist but I suspect that many of these criminals enjoy escaping justice more than the crime itself.
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