I honestly can't say I know if moving to opt-in would change police forces. I'd guess that military provisions of the GI Bill and training towards some tradecraft also help make the decision to leave easier. I wonder if there is something along those lines that could sweeten the deal to get people both in and out with less friction.
Just riffing now, let's say you add benefits the police officer can use after a term (versus only retirement type benefits that incentivize staying in) and some means of assistance into a new occupational field within the first term. It would seem the force also gets the added benefits of (1) more and a wider range of recruits, (2) workers who hate it have incentive to leave or at least don't feel/aren't trapped, and (3) just guessing, but I imagine the cost of the whole force goes down. Between a drop in average time in service of police officers due to more dropping out and fewer staying to retirement, the cost of the whole police force would drop. The new benefits' costs are an offset, though.
Edit: I wonder how an ROTC-type program would do. Make the rule police officers need a degree. That gets the level of officer up. The department pays for the four year degree and perhaps some living stipend, but you commit to an equal number of years on the force. As an office approaching end of that first term, you're making some new-ish public worker pay despite having a good Bachelor's degree (and no student loan debt). That gives incentive to leave.
Having to graduate first is a definite pro for the department, but it does run into problems if the recruit student fails a class, doesn't graduate, etc. Not sure how you deal with that. Maybe they work a lower level job
for the department while in school to help offset that.
Just riffing now, let's say you add benefits the police officer can use after a term (versus only retirement type benefits that incentivize staying in) and some means of assistance into a new occupational field within the first term. It would seem the force also gets the added benefits of (1) more and a wider range of recruits, (2) workers who hate it have incentive to leave or at least don't feel/aren't trapped, and (3) just guessing, but I imagine the cost of the whole force goes down. Between a drop in average time in service of police officers due to more dropping out and fewer staying to retirement, the cost of the whole police force would drop. The new benefits' costs are an offset, though.
Edit: I wonder how an ROTC-type program would do. Make the rule police officers need a degree. That gets the level of officer up. The department pays for the four year degree and perhaps some living stipend, but you commit to an equal number of years on the force. As an office approaching end of that first term, you're making some new-ish public worker pay despite having a good Bachelor's degree (and no student loan debt). That gives incentive to leave.
Having to graduate first is a definite pro for the department, but it does run into problems if the recruit student fails a class, doesn't graduate, etc. Not sure how you deal with that. Maybe they work a lower level job for the department while in school to help offset that.