I tried to have this discussion with someone last night, it didn't really have a conclusion, and I don't expect that it usually will. Like is said, the system is imperfect. However, imperfection and occasional corruption doesn't quite justify dismantling of the system. The dismantling of all police forces over these incidents leaves society with an "every man for himself" justice system, where justice is sought out with no real logic, precedence of reasonable punishment, or order.
To give the best analogy I can for the situation: No system is perfect, like I said. If you go to McDonald's, and you get the wrong order, do you demand the dismantling of the entire McDonald's chain of restaurants? Obviously, getting Diet Coke instead of Dr. Pepper (something that's happened to my cousin multiple times) isn't the same as unjustly getting shot, but my point is to say that the failures of few should not negate the proper actions of the many.
Oh, and since it was mentioned, and there's a clear correlation between the two incidents: I support the grand jury's decision, regarding Darren Wilson. There is reasonable evidence to support probable cause for the shooting, and while that might ultimately be the wrong conclusion, you can't simply throw a guy in jail because a bunch of celebrities are on Twitter calling it a travesty. "Innocent until proven guilty," might let criminals off due to lack of evidence, but I would much rather have that than a chaotic, "guilty until proven innocent" system, where you could be jailed for a crime you didn't commit because you can't PROVE each and every outlandish claim against you.
In this case, the cop deserves some serious jail time. I don't know about 20 years, that seems excessive, but overreacting and reverting to your training, even in an egregiously stupid way like this, shouldn't completely ruin and essentially end the guy's life. 5-10 years seems sufficient, though it's also hard to argue that nearly killing a man because you're a trigger-happy loon has a "fair" punishment.
Last point I just thought about while typing: Not to defend the cop's actions in any way (more an attempt to figure out why he reacted this way), but they DID mention a previous engagement where he was in a shootout. Perhaps this incident (stopped the guy, he got out, then reached back into the car) triggered some mental issue related to that previous encounter, and he violently overreacted, thinking the guy was pulling a gun. If that's the case, then I'd say that the system screwed up in not figuring out that h e was emotionally scarred from that event, but he would certainly still need to pay for his crime.