the second ammendment doesn't even really apply to the conditions of the world we live in today.
Keep in mind that stuff was written hundreds of years ago when we were just a group of rebels fighting for freedom.
The brits at any time could attack a settlement and it was necessary for people to be able to ready a weapon swiftly in self defense.
Criminals will always get guns regardless, true. But how often does a common man with a gun actually lawfully defend himself with said gun? There are many gun related deaths due to accidents. Especially along the lines of little kids getting a hold of their parents' guns and shooting each other thinking its a game.
Murderers will murder either way, but havng more gun control could prevent accidental incidents.
We don't live in a world where the average citizen needs to be constantly prepared for war anymore there will never be a time when our army is completely defeated and its left to regular people to fight.
Its really pretty impossible considering the sheer amount of soldiers that would come out of a draft.
But really, if the us was to fall it wouldn't be because of events on the battlefield, it would be because of economic hardships, which if history has showed us anything, the US economy tends to get better in war time because of our massive production of planes and stuff.