That's a very narrow interpretation of what it means to be in a Militia. You're using the text literally when one needs to look at the spirit of the amendment and what was intended when creating it. It wasn't just the threat of ones own government but the threat posed by other governments as well, since the United States was still a very small and very new power surrounded on two sides by a colony of the powerful French, and a colony of the powerful Spanish.
Additionally, the militia is designed to consist of every able bodied person whether they formally join an organized fighting force or just are on their own as a citizen and wish to defend their country and themselves from harm. The entire reason this law exist is because it is belief that Self Defense, is a right that people should have. What you are doing when you dismiss this argument is saying that the concept of Self Defense shouldn't exist, because there are people who possess the ability to command nuclear bombs. And frankly I'm not sure that's really a sound argument to make, though who knows, maybe it is.
With your other points, I actually agree with them and I find myself scratching my head at what exactly people wish to actually accomplish, aside from a mostly showy but pretty empty gesture.
If it's to stop these kinds of mass shooting incidents, well banning "Assault Rifles", which are simply semi-automatic rifles, that won't do much good. Handguns are and still are the preferred weapon of mass shootings. And it's not like rifle shootings weren't a thing before the AR-15 came to the market. The Texas Tower Sniper, The D.C. sniper attacks, Columbine, none of these would be prevented by this kind of ban.
If it's to reduce the amount of homicides by guns in general, again this won't do anything. Handguns are still the absolute number 1 contribute to gun crime, by a huge margin. Yet the majority of posters want to keep handguns but ban assault rifles, which would do little to curb crime.
If its to stop violent crime in general, well your actually a bit late to the party there. Violent Crime has been steadily declining over the past 2 decades, even during incidents of mass gun buying and selling, the rate of crime does not actually seem tied to the amount of guns currently in circulation within the United States. Gun Shows, where a lot of the iffy sales of guns occur, do not have a correlation with spikes in crime. Gun buy back programs do not seem to show an impact of violent crime either.
Frankly, at least to me, banning guns won't stop these tragedies from occurring nor will it make it harder for people to do them. So I have to wonder, what good will it actually do aside from being a political notch in one sides belt to claim victory over and get some good PR with?