Majority of convictions are plea deals, because trial penalty is just too large. And it is pretty much impossible to overturn existing convicion on anything but technical process mistake - no matter how convincing new evidence of innocense is.

While guilty murderer can walk, courts are also super eager to accept shoddy "scientific" evidence. And being defense layer is also worst for your career the being prosecutor. So, I am not sure it is as rosy as you make it sound.