False dichotomy and wishful thinking, all wrapped up in one!
If you disarm the law-abiding public, criminals will still have/purchase/make guns. Criminals can also use other weapons -
http://www.cnn.com/2014/03/01/world/asia/china-railway-attack/. Guns are essential to level the playing field. They allow anyone to protect themselves, regardless of size or physical skill (beyond marksmanship).
Getting into the debate over whether or not privately held small arms would be useful in repelling a foreign invasion or domestic tyranny really has nothing to do with whether or not private ownership should be legal. There is no guarantee that freedom of speech will save us from any calamity, but it is still our right. We have a natural right to self defense, just as we have a natural right to liberty in our thoughts and words. Guns are by far the best general tools for self defense and their legality is affirmed by the Constitution and Supreme Court.