You are stuck in the normal paradigm of cop arrests criminal -> criminal goes away for several years. But there is a middle step, DA prosecutes criminal, that went missing in SF. So instead, it's, cop arrests criminal -> DA refuses to prosecute -> criminal back on streets tomorrow. So really it is the DA that is responsible for 99% of the crime prevention effect of arresting criminals. Would you do your job if you knew that no one else on your team was doing theirs? That's what you are asking the cops to do.