It seems pointless to talk about conviction rates in isolation. In a perfect world, prosecutors would bring charges exactly when they had enough evidence to prove that the suspect committed the crime, and they would be found guilty at trial due to that evidence. One could also imagine a horribly imperfect world where prosecutors routinely charge innocents and achieve convictions in every case regardless due to kangaroo courts. In both cases, the conviction rate is 100%, even though one world is great and one is awful.

Without knowing why the rate is what it is, it seems pointless to talk about it at all. A 99% rate could be great, and a 50% rate could be great, depending on why it happens.