It would be fucked up if prosecutors weren't winning well over 90% of their cases, because being prosecuted is a punishment in and of itself --- it's ruinously expensive. So you'd very much hope prosecution is reserved for slam-dunk cases, as federal prosecution is in the US, where prosecutors also have a ~95% success rate.

They alternative, where prosecutors lose so many cases that their win rate goes down into the 70's or 60's, implies that prosecutors are taking lots of flyers just see how the cases shake out. That would be a tremendous abuse.