It's reprehensible that we require defendants to pay for their own trials. It's contrary to the very idea of justice to charge someone to go to court to defend themselves.
It's the same problem as with plea bargains, just with smaller quantities spread across many more people. The principle is the same either way: save us the trouble of proving you did what we're accusing you of, and we'll punish you less. The outcome is likewise the same either way: innocent people are coerced into admitting guilt because they fear being punished more after being found guilty in a trial.
(I like your idea too, of course. This is just another approach.)