Ignoring for a moment the obvious hitch that sperm does not automatically equal murder, there's also the problem that DNA forensics suffers from the same fakery problem that other criminal forensic techniques suffer from especially when samples are old and mixed https://www.newscientist.com/article/mg20727733-500-fallible...

DNA matching from a decades old mixed tissue slide is not an exact process and many factors from handling to a desire to close the case can lead to a wrong conclusion.

The most that can be said is that the detectives are satisfied enough to stop digging, not that the case is solved. Let's remember that a posthumous process allows for zero defense and criminal justice depends on defense.

And besides all of that, it strikes me as wildly unethical to burden the family with this information just to satisfy detective curiosity and desire for closure. I see zero benefit to going through the final steps after they determined that their suspect had already died. The corpse's ashes can't go to prison. The family just lives with this now. That's a negative outcome.