1. Nuke says that "Is superpower status worth it?" You are basically saying "We need a big army because we are a world superpower." Which avoids the point entirely. A better answer might be "The USA's superpower status is a historical/geographical legacy that cannot be avoided, and is actually worthwhile because it tends to prevent certain styles of government/imperialism."
2. Your assessment of world powers falls short of the mark in some aspects. For instance, you can hardly call Great Britain insular; and after it had stopped expanding, it chugged along just fine until shortly before the Great War, when all European powers started re-colonizing (cf. Rhodes).
There have been three great "superpowers" in the history of the world; political entities with a global might, reach and influence. These were/are the Roman Empire, the British Empire, and the United States. The first one was purely imperialistic, violently conquering for profit and gain, introducing civilization, but pillaging property and culture, and forcing its own culture on conquered areas. In many cases, this was not a bad thing, but in some, it was. Its downfall however could hardly be explained as simply as you did, or Mr. Gibbon would have been out of a job...
The second power was more of a modern state, in many ways. First of all, Britain introduced trade and prosperity to the areas in its Empire, frequently retaining only the most nominal political control, in exchange for economic control. It also lead to industrialization (as Britain was the first industrialized country) -- which ironically directly culminated in its downfall much later. There were relatively very few uprisings against British rule until the great nationalistic movements of the late XIX. and XX. centuries. In many cases, Britain also did not fight too much to retain control, realizing from America's example that a colony not directly under her rule could actually be more profitable for her via trade than if she had to pay for its upkeep. So what led to the British Empire's downfall? That is too recent an event to have gotten too much attention in the history-books, but anyways I will attempt to summarize what I have gathered. First of all, the Great War, which due to the crude methods and scale with which it was fought, slaughtered the flower of a generation and left little resources to pay for and rule the Empire. This was the turning-point into the modern age, at least partly. The second point which completed modernization was the Second World War, epitomized by Churchill's words about secondary-school students, something along the lines of "They have defended their country; they have earned the right to rule it." (paraphrase) Britain was in no shape after these devastating wars to continue her Empire, and due to nationalism, her Empire didn't want to continue either. So gradually most areas declared independence, went through a period of turmoil; then realized that trade with Britain and British citizenship was a good idea after all, and re-joined the Commonwealth... It wasn't a simple matter as you seem to portray.
The world events from 1914 onward showed that the United States was the next global power, and she showed her mettle in both world wars and in the subsequent Cold War. Her involvement in world events has defended and protected causes which posterity has shown to be just, and she is a worthy successor to the British Empire, if not in precisely the same role.
Modernization has made the thought of empires in name unbearable to the average citizen, although they are here just as solidly in an economic sense. So in the modern age, we must separate military dominance and economic dominance. The USA currently leads the world in both. Many people speculate that Indochina will overtake her in an economic sense. That remains to be seen. If it is so, all the weapons in the world will not help the citizens of the USA in keeping up with living standards elsewhere. But that is very far in the future if it will happen at all.
In a military sense, however, the USA's dominance certainly must be preserved for the peace of mind of many small countries around the globe, yes, in my opinion.
Oh and Freen, Spain was never a global power. Nor was China. But basically I've tried to counter your statement that "they fell because they stopped expanding their army and thus lost control of their economy, which was based on foreign investments". In the modern age, cash flow has largely become separated from military might. In a way, Talleyrand was the precursor of that.
[edit] To Urran's post below: Boy, you sure have been reading this topic attentively.






