I get what you are saying, and agree with your decision to not talk to the police, but it isn’t true that there can be NO upside to talking to the police.
For all the bad things police do, they DO also solve some crimes and arrest some dangerous people. If someone killed one of my loved ones, I would hope the police would be able to find, arrest, and successfully prosecute the murderer.
If no witness will speak to the police, it makes solving a homicide much more difficult.
This is why the current approach to policing is so awful. We should work on fixing the system so talking to the police is a good thing.