Police can feed cold-case DNA to these DNA registries also, and get back matches like, “the possessor of this DNA is a close relative of this person who is in our database.” Then the cops just need to figure out which close relatives of that person were in the right place at the right time to commit the crime.
Some people are insufficiently paranoid about stuff.
OTOH, one of my great grandmothers took the secret of her children’s father(s?) to the grave with her, so these registries might be a way to connect with an unknown side of the family. I’ve never thought about the fact that this might be an unwelcome surprise for my great grandfather’s family. The events in question happened in the early 20th century, but if he and his legitimate progeny lived normal lifespans, there are certainly people alive who knew him.