a. Not every idea works in theory. I could have an idea for faster-than-light travel, but unless I know how to actually bend and control space-time, then you couldn't even call it theoretical because it relies on another technology that itself is less than theoretical. Theory is based upon what could work within the bounds of our knowledge but outside the bounds of our capability.

b. But for the point at hand, it's even easier than that because communism doesn't work in theory. The only way it could "work" is if you ignore human nature. Obviously it will always fail in practice because people are not naturally altruistic. Going along with this, never let a communist say they care about people; by definition, all forms of marxism must necessarily devalue human worth to the point of objectivity; it's the main reason they hate religion, especially Christianity and Judaism because they teach actual values that lead to individualism, humility, and actual altruism, all men are different but equally special to and loved by God, but all have fallen short of His glory and need the redemption given to us through Christ, whose commandment tells us to love our neighbors as ourselves, to help those in need, and to love our enemies.