Criminals are affected by supply aswell. You take away the law abiding citizens guns and you take away a large portion of illegal gun supply, sure criminal enterprises will see more action, but the availability will certainly drop. Also how do you defend yourself with a gun if the criminal has a gun pointed at you? I don't assume most criminals call ahead and inform you that they are going to attack you...
My point is that while we can't stop all criminals from getting guns we can certainly stop some of them from getting them, and thus reduce the amount of situations where criminals with guns use them against law abiding citizens.
Also if everyone has a gun won't people just shoot you first and ask questions later, because they assume you will shoot them if they don't?