To provide a rather different perspective, I was born and raised in Alaska where pretty much everybody has several guns. Granted, many of those guns are designed more for killing bears and moose than people, but they'll do the job, and most people have a combat-oriented pistol or two. I got a rifle for my fifth birthday.

I'm shocked that people litteraly sell knives designed to hurt people on the internet, and that people on HN come to the defence of these arms dealers.

Most of the knives in question are general-purpose utility knives. They are not designed to hurt people, and that is not the reason most people own them. I carry a Kershaw Leek myself; it has a 3 inch blade, assisted opening, a frame lock and a pocket clip. I routinely use it to open packages, trim rough edges off various objects, cut rope, strip wire, scrape contaminants off surfaces and remove those nasty little sticky screw covers from laptop computers. I have studied and practice the use of a knife as a weapon, but that is not a significant reason I carry a knife, and I would carry a different knife (or more likely, a pistol) if it was.

I've been to the UK. I did not bring my knife because I knew it would be illegal to carry there. Turns out, it's probably illegal to own there, and the multitool I usually keep in my backpack might be illegal to carry in public because it has a locking blade. I never really noticed how much I use my knife until I started going to Europe regularly and couldn't bring it along. I'm constantly reaching for it and finding it missing. I've been carrying a knife of some sort since I was 12.

Since then, that government has entrenched power to the point where it would be nearly impossible for even another state to take it down, let alone a group of armed civilians - thus rendering the constitutional clauses irrelevant to their original purpose.

Aside from the fact that some Americans aren't so happy about that, I think you're mistaken. A large minority of the civilian population would have a much easier time deposing the government and instituting a new one than a foreign state (or group of US states attempting to secede) would. A foreign state attempting to overthrow the US government would have to deal with a fully committed US military and a large portion of the civilian population, which, as you noted is well armed. A large minority of the population rising up against the government would likely gain the support of some of the military and could blend in with the rest of the population when not fighting.

In the rest of the developed world where weapons are generally banned, as they are in the UK, criminals simply don't use weapons on civilians because the penalties for doing so are so harsh

I don't think that's the reason. Penalties for being a felon in possession of a firearm in most US states are severe. Having a weapon while committing a crime, even if it was otherwise legal to have the weapon and the weapon was not used in the crime usually comes with a significant mandatory minimum prison sentence. Firing a gun during another crime means decades in prison.

Criminal-on-civilian violence actually is pretty rare though; the vast majority of violent felonies in the US are criminal-on-criminal. The danger to civilians from Johnny the drug dealer shooting Jimmy the drug dealer is small.