I believe that the desire for more firepower doesn’t really go away, so there is no reason to expect a freely chosen self-limitation to the arms race between “criminals,” “law enforcement,” and “law-abiding citizens.” (Or alternatively, since this is the USA, the “race arms race” between “persecuted” blacks and “law and order” whites, who fear and demonize each other.) The law is not there to say, “Do what you were going to do anyway.” The law is there to act as a limit on people’s behavior where they would not limit themselves.
Guns would be collected by law enforcement and melted down.
As for, “new ones from coming in from Mexico or even Canada,” at the moment the bigger issue is the USA exporting illegal guns to Mexico. If we get to the point that Mexico has more of a gun trade than we do, I guess we’ll have to cope. Personally, I’d be glad to stop the flow of illegal arms to Mexican organized crime, something the USA seems unwilling or unable to do effectively at present.