There are also hoops to jump through in Illinois. Some normal people just snap. So unless there is a way to prevent all gun possession (any gun, any where, by anyone) then preventing law abiding citizens from possessing a firearm makes them sitting ducks.
Guns do, virtually, last forever. Any gun in existence, if maintained, will last until our grand children our dead.
Also, I believe gun crime goes immediately after gun bans, but goes up once people find new sources for guns. Guns are ALWAYS going to be available to criminals, there are no two ways about it.
It doesn't work in a single state, obviously, because there are easy sources all around.
Guns can last a long time, but that's not the point. The majority of guns will eventually fall out of circulation, is the point. I doubt that most guns are properly maintained and whatnot, and that's kind of the point.
Your own post proved that gun crime went up immediately after. When you get into trafficking and smuggling, you suddenly exclude the vast majority of people, especially petty criminals and normal people who snap, from the equation. So it depends how you define "criminals" -- yes, organized crime and the like will still have them (and they do in my city), but the average person/criminal doesn't.