If the outcome of the plea bargain is about what the defendant would expect after conviction on reasonable charges, plea bargaining would be fine. But that's fantasyland.

The problem is the incentives are set up such that the charges will not be reasonable. They can't be reasonable, since the state can't possibly afford to take more than a tiny percentage of cases to trial. Which sets up another incentive - the state must make an example out of people who demand a trial to encourage everyone else to take the plea.