Using other forms of intervention (obviously for non serious crimes) clearly has some impact- otherwise they wouldn’t do it would they?
You naively assume that the goal of the police in such a circumstance would be to have some impact on the criminal.
They may have other goals, such as avoiding race riots or racial tension, or just minimising their own workload.
Also
There are four goals of the justice system, and you seem to only consider the last one on this list, though you haven't clearly spelled out what you mean by words such as 'result' and 'impact'.
- Punishment by the state, i.e. satisfying the people's need for revenge
- Deterrence, both for the individual being sentenced and others who see sentences passed and wish to avoid the same fate
- Public safety, i.e. directly preventing further crime by the same perpetrator by locking them up
- Rehabilitation of the criminal to prevent further crime from them.
Going via community elders to deal with an offender might have a greater impact on the offender's rehabilitation (goal 4 above), but does not satisfy goal 1, and has a negative effect on goal 2 - the individual will see themselves as immune from the law, that the police won't touch him.