Actually, that's not a flaw in his logic, but a flaw in your understanding of how the world works. Very few entities actually cease to exist, in legal terms. Usually, someone claims successor state status, which causes them to assume both the rights and obligations of the entity they succeed. For example, Russia is the successor state to the Soviet Union. Austria and Hungary were both successor states to the Habsburg empire. West Germany assumed successor status for the German Reich (and hence legal responsibility for the acts committed by Nazi Germany), and then for the GDR after reunification. And the UK most certainly would be responsible for the British Empire, long gone though it is.