1. You can't remove all guns, you can only remove the legal ones.
2. If you had the ability to remove illegal guns from society you would have had the ability to remove the criminals in the first place.
3. Even if you did remove the millions of guns and prevent manufacturing / imports (even though you can't for drugs) you don't know for certain murders will go down. These people are ruthless. Jumping people with machetes is always an option. It may be worse since guns are an equalizer and let people defend themselves. You're a lot less likely to run up on someone when you know they have a gun.
The argument isn’t that all murder stops. It’s that it goes down, dramatically when you remove guns from the equation.