We can't bring Nicole Simpson back with DNA evidence - nor could we keep OJ off the streets with it.
Your observations about DNA and the O.J. murders are noted but I would counter that there seems to have been 21 people who were scheduled to die over the past three years who were cleared to some extent by such evidence and taken off death row. That is 21 people out of thousands no doubt and probably statistically insignificant but what is the measure of a wrongly convicted life?