Statist: Libertarianism doesn't work. If it did, why are there are no libertarian countries?
Libertarian: There are no flying butler robots that clean my house, but that doesn't mean that it won't someday be possible. A libertarian society has not been tried, but that doesn't mean it doesn't work. Someone once had the idea of using forks to cut food. Would it have been a valid argument to say "Well, forks have never been used, so they won't ever work?" No, because something has to be tried for it to work and can't be dismissed as not working without proof that it doesn't work, and I have never talked to anyone with a valid reason as to why a libertarian and/or anarchist society wouldn't work.
Statist: Oh, I just thought of a libertarian country: Somalia.
Anarchist: A failed socialist regime with six governments fighting for power is not a libertarian paradise.