Emperors and Constitutions: Illusions of Continuity?

A quirk of Japanese history is the survival of the Emperor, for over two thousand years. This, despite multiple violently-ascendant governments in that interval, most of which relegated the Emperor to a figurehead who spent his days in a pleasant court in Kyoto writing poetry while the military rulers ran the country. This seems strange to just about everybody outside of Japan. If you take over a country, the first thing you do is stamp out all vestiges of the old order, right? Especially the figureheads of the old authority!

There may have been a method to their madness. Once the institution of the Emperor had survived one or two of these changes of the guard, keeping the Emperor around - with no power or ability to muster forces of his own - might make sense. It gives a false sense of stability by presenting a continuous succession of figureheads, giving the new government immediate legitimacy.

A cynical view of the longevity of the United States Constitution might stir similar thoughts. The democracies of the world frequently throw out their previous constitutions and write new ones even without violence, often multiple times per century. In fact, imagine for a moment that there is a European country that has kept the same document, unchanged, since the eighteenth century. Certainly this would seem curious; and the government in question, dishonest about how they're executing this ancient parchment, or (more charitably) maybe they're just a rural backwater where nothing much has changed. Certainly this latter situation does not obtain in the U.S. It might be the case then that the true function of the Supreme Court is to interpret challenges to the U.S. Constitution in whatever ways create the fewest ripples with respect to modern sensibilities. Activist judges or not, it would seem that "living documents" guarantee a certain amount of non-elected legislating.

A related question would be the relationship of currency stability (say, month-to-month fluctuations) over time relative to constitutional turnover. Do countries that explode their parliaments or set up new governments have less predictable currency values over time?