It is all part of what is known as the 'Adversarial System' that we use for our legal system. There are lots of arguments for why it is used, and in fact, one of the main purposes is to mitigate that kind of 'tribalism' you describe. The alternative system is known as an 'inquisitorial system', and puts the state in charge of everything. By explicitly acknowledging that the defense and prosecution are working against each other, you can help make sure that the motivations are clear and not misleading.
Adversarial is good. What the OP may be suggesting is: take a pool of lawyers, and have them be either prosecutors or defense lawyers, on any given day.
It has some practical issues (defense lawyers are not paid by the state, prosecutors are).
A robust adversarial system can be good. What most folks experience in the US isn't a robust system that is anywhere near fair to both sides.
Many defense lawyers are paid for by the state. Not all, but most. But they aren't really paid equally in most cases: Defense lawyers provided by the state tend to be overworked and not have enough time to adequately help folks. I'm not sure anywhere has laws that mean they get as much money and time as the prosecution has to put towards a case. They aren't paid on par with private defense lawyers either, unfortunately.
An adversarial system is only good if both parties have similar support. When one side is obviously disadvantaged when the defendant is too poor to hire a lawyer outright, the benefits of the adversarial system are fewer.
From what little, and indirect, contact I've had with the US criminal justice system my impression is most defense lawyers are paid for by the state (e.g. the public defenders office).
A quick search turns up this[1] report from 2000 that says:
Over 80% of felony defendants
charged with a violent crime in
the country’s largest counties and
66% in U.S. district courts had
publicly financed attorneys.
Having said that, the report is from 2000 and it seems the data is from the mid to late '90s. So things may have changed since then but I don't see any evidence of that.
I'm not sure that combining the prosecutor's and public defender's office would work, but having a system where the lawyers end up switching between prosecution and defense cases does seem like it may help avoid pushing them into extreme views for either side.
It's funny that this suggestion seems so radical considering that a somewhat related discipline, debate, requires participants to argue both pro and con throughout the course of a competition. I remember realizing that by arguing both sides of an argument, it was forcing me to concentrate on the mechanics of argument rather than the actual topic. It didn't matter which side of the argument was better, only which side was argued better.
Similarly, I'd expect lawyers who were forced to alternate between prosecuting and defending to concentrate more on the mechanics of the process and less on the result.
Competitive debate is a performance for a self-selected audience. Voters and especially defendants are more interested in the results than the mechanics.
Do you really want your criminal defense to be handled by someone for whom up to 2/3 of their job may require close cooperation and good working relationships with the police?
There is a difference between former prosecutors (who have knowledge of a prosecutor but not the political and institutional incentives of a prosecutor) and someone who much of their current workload is as a public prosecutor.
Yes, a former prosecutor is a great person to get as a defense attorney. That's not the question I asked.
You asked a question about a hypothetical, but I don't know why a relationship with police should necessarily inhibit good representation.
That said, it's not like the defense/prosecutor caseload would be the only thing to change if this hypo came to be. It wouldn't happen in a vacuum, no "suppose everything was the same tomorrow except everybody's calendar got mixed up!"
Well, sure, if we enter a world where criminal convictions aren't politically useful stepping stones in the way successful criminal defenses aren't, the plan is great.
To me, it's kind of like, "know thy enemy." If you've worked with them, you know their dirty tricks. If they know they're going to be up against you, they may be less inclined to use them because they know that you know what they're up to.
Actually, most defense lawyers are paid by the state too:
> At felony case termination, court-appointed counsel represented 82% of State defendants in the 75 largest counties in 1996 and 66% of Federal defendants in 1998.
https://en.wikipedia.org/wiki/Adversarial_system