The British Empire's expansion was driven by trade protected by the RN, the biggest spike being immediately after the Napoleonic War. It was never one of pure military dominance or expansion.
The pride of Empire, the Raj was run as a private enterprise by the British East India Company until the mutiny of 1857 (regular army took over), basically they were the original PMC (forerunners of Blackwater / Armor Group).
The Industrial Revolution drove the expansion, UK mills needed overseas raw materials hence entrepreneurs struck out to establish trading posts, which when threatened were protected by gunboat diplomacy and eventual forced seizure (Hong Kong post Opium War)
The British Empire didn't collapse overnight it retreated and became the Commonwealth. The UK was bankrupt post WWII and the colonies demanded self determination.
Reference America's Empire, the countries decline will be marked by an ideological change elsewhere. The concept of western democracy being superior may be usurped by the idea of benevolent autocracies. The Chinese model of tightly controlled government, which allows limited freedom under a micro-managed system may end up be the favoured approach by those nations looking for a rapid economic expansion, which both satisfies the masses and allows for stable growth. The relative decline of the west and the growing economic success of the east may sour the American dream, nations may start lookong for other models of governance and cultural influence.
Hmm, some confusing sentiments. Again its not as if the Indian subcontinent 'invited' the British, far from it, and their rule was maintained not by offering citizenship or enticing them with the wonderful benefits of western civilization, but by military force, thus the revolt. To characterize gunboat diplomacy as a response to "threatened" trading outposts is also a bit strange. The Chinese demanding that you stop importing Opium is hardly "threatening" yet again the outcome was seizure and military dominance, (Macau, Hong Kong.)
Quite frankly the British Empire, at least in Asia, did collapse overnight. The Japanese swept the British from Hong Kong, Singapore, Malaya over a matter of weeks. British power and prestige in Asia was irrevocably damaged, especially in a region known for prestige and face. This was not a matter of money, the British had been shown to be less than invincible and in a system of Empire of Core and Client states this was a death blow. To say the UK was bankrupt does not preclude their military fatigue/defeat, in fact their bankruptcy was a direct result of fighting wars against revisionist powers (The Axis Powers.) If you can point to the UK attempting to create international institutions to safeguard their power at a later date then by all means show me.
To argue that America will eventually decline (in a relative sense) misses the point. Such cycles are inevitable. This is why the liberal international order was created, China is a member of the UN, IMF, WTO and benefits immensely from this arrangements, they are in fact the sole reason China is able to modernize as it is (access to the global economy.) The more that China (or other revisionist powers) partake in the system, the more bound to it they become eventually being molded into 'responsible stakeholders.'
Realism as such, while being a powerful undercurrent, is sharing space with a growing 'community' of nation states that are bound together in a growing web of interdependence. Do you not see what the U.S has done? It has escaped the inevitable cycle of 'Empire' by creating a system that treats all nations as equal despite relative hard power discrepancies.
To argue that perhaps Chinas socio-economic model presents an alternative is a flawed analysis.
a) Chinas recent growth and prosperity are the result of moving away from command and control autocracy and towards liberalism, not the other way around, this recent growth is also contingent on them partaking in a liberal international order.
b) From the perspective of economic systems the Chinese are in their infancy, a command control "micro management" system can work well when engaged in rapid industrialization, but thats all it is so far , 'playing catch up,' low value added manufacturing (the Chinese add $2 to every ipod) will only get you so far and the Chinese are well aware of this. As the economy matures, growing middle class, and the division + specialization of labor increases, micro management will encounter the usual deficits that have been seen in these discredited systems time and time again.
c) By drawing China into the global economy, China has been forced to 'open up' to a degree. While soft power analysis are flimsy, the power of culture and interdependence is noticeable. To the degree that modernizing, and becoming more 'western' seem one and the same thing. The similarities between Shanghai and New York these days is startling, as is the uniformly similar fashion of citizens within. Such is the nature of this open and free global exchange that people are no longer content with conditions within their own domestic borders, especially within those borders where populations are repressed.
The Chinese leadership is painfully aware of all of the above and no where will you find them saying that Democracy will never come to China, they are simply concerned with managing the transition across an incredibly large fracticious land mass.
The question is why would anyone attempt to re-model the global system when they benefit immensely from said system? Barring global economic catastrophe, it is simply not in their interest to 'dethrone' the US and the liberal international order.