> On criminal trial, not in prison.

In this case they ended up getting guilty pleas in exchange for 'promises' of leniency. That sounds like 'putting people in prison using leverage' to me. With an imbalance between resources of the entity going after you and what you have available, I would say the giant corp will win the overwhelming majority of the time.

The problem is that they have every incentive to push people to pay them/take a deal with what is essentially extortion and there is no oversight. If the city prosecutor were threatening otherwise upstanding citizens with jail time if they didn't pay them many thousands of pounds for mystery expenses viewable by only the prosecutor, that person would be impeached or at least lose the next election, then be prosecuted for corruption. No such luck when the entity pursuing is 'faceless corporation that needs to maximize profits and has no person held to account for injustice'. I fail to see how this is in the public interest.

I mean, many others have used the same tactics with civil suits—being stripped of all of one’s money might be marginally better than being put in prison, but I’d argue it’s still on the same spectrum, so it makes for a hell of a lot of leverage.

So I don’t think this works as an argument against private prosecutors—it’s rather an argument against a legal system that necessitates ruinously expensive lawyers, which is a major problem, but one I see no solution for, not even a blank-slate one. (To be clear, I still reserve my judgment on whether private prosecutors are actually a good idea. What do you do if a private party takes up a murder investigation and screws it up?.. I’m not ready to give up non bis in idem for this. On the other hand, the idea sounds sufficiently reasonable that I probably wouldn’t have thought to question it had I been brought up to think it were normal.)

> I mean, many others have used the same tactics with civil suits—being stripped of all of one’s money might be marginally better than being put in prison, but I’d argue it’s still on the same spectrum, so it makes for a hell of a lot of leverage.

So if one thing is terrible let's use that to justify something worse?

I’m just not convinced they are different things. Instead, what you’ve said sounds like a general argument against allowing people (and other legal entities) to bring others to court.

A prosecutor has broad powers to force a person to defend themselves in court and take a chance that they may lose -- this is incredibly coercive.

You are letting a profit motivated entity hold those powers over its own contractors in order to threaten them with lengthy jail sentences unless they agree to pay back the contested money and plead guilty to a lessor crime (which then makes it impossible to ever assert you were wrongly convicted and shouldn't have to pay).

To add to this, a corporation is does not have a corporeal presence that can be held to account for doing something egregious -- they just get fined and anyone involved almost always says 'I was just doing my job, a cog in the machine' and that's the end of that, whereas if a public prosecutor was extorting the community to pay them large sums of money or face an uncertain trial where they cannot access the evidence needed to prove their innocence, then that prosecutor could be put on trial themselves.

> this sounds like it could somewhat mitigate the phenomenon

Wouldn't work in the UK; the Director of Public Prosecutions (a member of the government) has the power to take over a private prosecution, and then drop all charges.