I think few people would argue against the benefits of genetic testing, but just because it's a great thing doesn't mean that being complacent around adjacent shady business transactions is the right approach.
You can be for it, but also for privacy, protection against insurance discrimination, etc. at the same time.
I’m dead in the long run regardless, and would rather find every way I can be impactful with the time I have left, even in these small ways.
10 million tests vs 10 million person database means 1 in 10 trillion false positives happen 10 times.
Generally at a 7cM overlap you have 50:50 chance whether you're related to that person or not. Anything over say 150cM you're virtually certain to be related (ignoring endogamous populations). At 3,400cM (full-match) it's not even worth considering randomness.
Paternity tests involve random ~50% of your dna. Sperm samples are generally contaminated and thus less clear cut. But, people’s relatives have been used in the past for these tests dropping the bar even further.
However, being related genetically is not enough. Some people have twin siblings they don’t know about pointing out being related is not nessisarily mean you know anything about the other person allowing for false positives even at 3,400cM.
Unlikely sure, but harm comes in many forms. Saying I can’t think of anything is a long way from saying it’s safe.
If you've got a identical twin then you're right that their child will be genetically indistinguishable from your child. But that's down to the nature of genetics rather than anything to do with the reliability of testing or database size.
If a test failed because _ then it failed independent of why it failed. If your long lost twin causes you to be involved with a paternity suit or homicide investigation then that's both a false positive and a significant problem.
Suppose a male with red hair, green eyes, and AB+ blood actually has those traits they are just not enough to unlikely identify someone. Adding more genetic traits on it's own is not enough for example you have identical twin or even triplet separated at birth. The core issue is DNA marks are not independent though people often assume they are.
Sure, at some point in the future it might get fixed or always fail, but until then you need to deal with today not some mythical perfect future.
I think the pros far outweigh the cons.
Sure, 23andme and GSK and others will get richer off this but so what—they are doing something good.