That is a coincidental match, though! It's the risk of a false positive when you compare exactly two samples.
The problem is that we're proposing a DNA database, so you're now comparing 1 sample found at a crime scene with 300ish million people. That's a 300M in 100B chance, or 1 in 333, of some person having a false positive match.
Now combine that with thousands of samples being tested every day (because let's be honest, why wouldn't you use such a database for literally every single crime?) and you're looking at dozens of false-positives per day. It is only a matter of time before it isn't someone with an airtight alibi at the other side of the country, but someone reasonably-close who was just sitting alone on their couch at home all weekend. How are they going to prove they weren't involved in the crime?
This is why large-scale DNA matching is a horrible starting point for a conviction. It gives you a false sense of security, and it makes the investigators lazy. In most cases it is far better if DNA is used as the final nail in the coffin when you've already got a solid case against a suspect.
The problem is that we're proposing a DNA database, so you're now comparing 1 sample found at a crime scene with 300ish million people. That's a 300M in 100B chance, or 1 in 333, of some person having a false positive match.
Now combine that with thousands of samples being tested every day (because let's be honest, why wouldn't you use such a database for literally every single crime?) and you're looking at dozens of false-positives per day. It is only a matter of time before it isn't someone with an airtight alibi at the other side of the country, but someone reasonably-close who was just sitting alone on their couch at home all weekend. How are they going to prove they weren't involved in the crime?
This is why large-scale DNA matching is a horrible starting point for a conviction. It gives you a false sense of security, and it makes the investigators lazy. In most cases it is far better if DNA is used as the final nail in the coffin when you've already got a solid case against a suspect.