With regards to cultural issues, I think that the death of religious conservatism as it stands is unfortunately exaggerated in the long run, but when it comes back it'll be a bit different from the way it looks now. The power of religion has generally been cyclical across the world, as opposed to simply being a parade of "people overcome the chains of ignorance". During the peak of the Enlightenment, it wouldn't have been completely unreasonable to argue that, at least for the somewhat educated peoples of Europe, religion was becoming irrelevant. Yet by the Victorian era, at least in Britain and the United States, religious sentiments were stronger than ever (albeit in a much different form from modern Evangelical Christianity common in the US these days). Islamic Fundamentalism of the ISIS/Al-Qaeda/Taliban variety only arose in the 19th century and only gained political relevance in the late 20th century, after decades of relative secularism. "Social conservatism" only really became a powerful political force in the 1980s, and at one point it was argued that the Reagan coalition was strong enough to basically hold power forever, which really goes to show how hard it is to predict what trends will hold past twenty years or so into the future.
Anyhow, America's "glory days" are mostly built on it having a combination of a lean government, an educated populace, lots of very hard working immigrants, and the alternatives generally having some sort of "fatal flaw" that made the US competitive towards other industrialized or semi-industrialized countries that would otherwise have outstripped it. It had no massive overseas empire to maintain like Britain, no large population of illiterate peasants in need of education like Germany or Russia, and it wasn't in a state of constant political turmoil like France. The US had many problems, massive corruption in government being one of them, but it was generally able to overcome these issues simply by being the best option for people. Your average lower class railroad worker making pennies wasn't well off by any means, but he was better than if he stayed to starve to death in Ireland or get shot at in Central Europe, and the capital accumulated by increases in productivity and savings allowed for the creation of a large middle class. Further, after two World Wars, the US benefited greatly from having an intact industrial base after most other countries had been devastated and/or been taken over by Communists. Thus, even as the US began to lose the advantages it started out with, it was still a competitive option simply because other countries made the same mistakes and then some. The seeds of American decline were laid decades ago, they were just cloaked by the leading "alternative" to the American model being Soviet Communism.
These days, America doesn't have the advantages it once held. The US maintains a gigantic world-spanning empire that far outstrips the former British Empire, and judging by the range of views held to be acceptable by the American media, that empire isn't going anywhere until America becomes incapable of maintaining it economically or militarily. At the same time, it also maintains a gigantic welfare system that somehow becomes less and less efficient as more and more money is put into it. It has a convoluted tax code and regulatory system that is only beneficial to the sort of massive corporations that see the costs of a phalanx of lawyers as a drop in the bucket. Where most Americans were once self employed, now starting a business is so unrealistic for most that there isn't an alternative to working for a small subset of corporations for most of your life. In turn, this binds people to their jobs, letting businesses get away with quite a bit more than they would otherwise since the threat of being replaced is over the heads of the workers. All of this creates a sort of stagnation in American industry that causes it to become less and less competitive on the world stage. Why should my pharmaceutical company produce cheap, effective medicine when doctors are basically required to recommend it under fear of tort lawsuits, the FDA prevents competition from anything short of a multi-billion dollar company, and patent law grants me a monopoly of several years that lets me charge a thousand dollars for something that costs a few dollars to manufacture? Why should my energy company invest in improving efficiency or alternative sources of power when I can use eminent domain to seize valuable land for my benefit in America and the CIA/US Armed Forces to seize it abroad? Why should my bank operate carefully when I can take massive risks, have my assets artificially inflated through careful intervention of my friends at the Federal Reserve, and then be compensated by Congress if things go badly anyway?
The US government, despite always being decried as far-left or far-right, has been extremely centrist for a very long time. A politician can argue how the US should intervene in a conflict halfway across the world, not whether it has any right to do so. Arguing against corn subsidies or any other form of subsidy (direct or indirect) is more or less political suicide. The scope of the healthcare debate for most is between the Republicans advocating the present, very broken system that they claim constitutes a "free market" and the Democrats advocating a mandatory wealth transfer from young, healthy Americans to insurance companies cunningly disguised as "universal healthcare". If you try to argue that the ACA isn't universal healthcare you'll be quietly moved to the peripheries of influence, and if you argue that the current system doesn't even remotely approximate a free market you'll be outright ignored. You can argue about taxes, but again in limited terms. You can "tax the rich", but you'll be raising them on the middle-upper management types and the few remaining entrepreneurs since the sorts of people that actually matter in the US don't have an "income" to tax and make their money on diversified investments and government debt besides. So the biggest range of debate is on topics that are basically irrelevant to such people, like whether gay people should be allowed to marry legally in a country where the concept of marriage is becoming less and less relevant, or whether abortion should be legal in a country where birth control is easily accessible.
Really, the only thing holding up American supremacy these days, besides straight up military might, is the standard of backing currencies (and oil purchases) with the US Dollar, and that only lasts so long as US debt is worth something, which might not be so long as the American government thinks.