Abolishing supplementary investigations is only possible when you fully trust the police, like in the Jang Mi-ran case

If the police close a case, doesn't it mean the police will be the ones conducting the reinvestigation no matter how many requests for a re-examination are made?
Ultimately, if the police try to close a case to help a colleague or to protect themselves, is there any way to stop them?
Under the guise of "stripping prosecutors of investigative powers," isn't this just increasing the damage to citizens who have frequent direct contact with the police? I worry that, much like the National Election Commission, they will only end up being better protected even as they become more corrupt.
It is a relief that the Jang Mi-ran case has come to light now, but I am worried that after the abolition of supplementary investigations, such cases will be buried forever.