It's the most powerful self-defense arm available to date. It's impossible to magically get rid of firearms, and criminals do not follow the laws as it is, so they'll just continue to own them(Prime example prohibition era 1930, s when alcohol was banned in America, all it did was put it in the hands of criminals and create a monopoly which allowed the mafia to rise to power and wreak havoc across the country). Whether or not there is a law that stops people from owning weapons doesn't quite change the fact that life is fundamentally unsafe; the moment you set outside your house you enter the external world and all of its uncontrollable forces(such as nature and the actions of other people). If it's not guns people will just go to knives and other weapons, and no one will ever hold accountable the wielder of the weapon who is the common denominator in this equation.
Perhaps the most critical problem with gun control is basically the flawed idea that it's better to trust those with power in contrast to your equals with their own free will. When, in reality, individuals have a much better chance defending themselves from another citizen with a gun then an over bearing corrupt government with much more powerful artillery and crime syndicates that benefit off of strict laws that strip law-abiding citizens of their rights.
Ideally, no guns would exist or better yet a world without violence, but this is fantasy. A life of freedom calls for an element of danger and unpredictability, and people must not be forced to rely on others for protection instead they must be able to stand on their own two feet unlike sheep waiting to be slaughtered at pasture.
Could you show me your source and proof of this comparative data? Also please keep in mind that there may very well be alternate explanations for the decrease in gun violence (for example, example people could switch to knives meaning the same level of violence still exists but shifted into a different form). This would of course mean that the fundamental issue isn't guns(shocker) it's that some people aren't taught the value of life/have severe anger that has been left unchecked and results in the loss/harm of innocent lives. That said the real solution is creating an environment where people can seek help before it's too late without being ridiculed and instead applauded for seeking help instead of hurting innocent people.
Again, you can't magically make guns disappear. If it were that simple weed wouldn't be sold illegally, neither would alcohol during the prohibition era. Fundamentally, Criminals by definition do not follow the laws and will continue not to that's what makes them criminals and not law-abiding citizens.