The "the trajectory of the US on a 'Chinese timescale'" barely even registers. While the CCP might not be that old, China considers itself an ancient society with a level of historical coherence that makes the United States rather insignificant as a lesson in political history.
You've offered a hugely reductive and rosy view of American history. Democracy is easy until it gets hard. America has fought a civil war before. The Congress basically does not function anymore ergo the "imperial presidency". The balance of powers the framers created when the country was a sparsely populated periphery of 3 million people has changed dramatically. George Washington rejected a partisan affiliation and regarded it as a societal disease. He was elected unanimously. Without his personal touch, the country probably would not have survived into the 19th century.
America has come into superpower status as the rest of the world destroyed itself. Post WWII, the quality of our politics have steeply and sharply declined, go back and watch debates from the 1960s. The same civil war cleavages are being debated today, with far less eloquence than a Buckley Jr or a James Baldwin.
People outside of the United States seem to imagine the rest of the world is naive about democracy. No. Rather when you are an old enough society, you experience more things and understand democracy has its own risks - just as any form of government. Democracy is not as easy or "self evident" as Americans think. The nature of American democracy has already changed radically since the founding. If you factor in "universal suffrage" to mean "democracy" then the United States hasn't even existed as a true democracy for 100 years.
Are you really that bullish on the United States a century from now? Based on what? It seems obvious to me the next century will necessarily trend towards multipolarity. The chances of any American being born today having a better life than their parents have to approach the chances of techno-utopian fantasy. Which will only be attained if Americans learn to live with China as a partner not an adversary. Chinese scientists may cure cancer. They will innovate and solve other problems for the good of all mankind. Meanwhile in the United States your politicians can't even achieve universal healthcare. That is just one example where the United States has continually failed to make change at pace for decades. WWII recedes farther and farther into the background. So does the Pax Americana. China is not the enemy of the average American. It's just an obvious smoke screen for the homegrown problem - greedy elites robbing the country blind, buying elections and achieving corruption at a scale that makes "democracy" seem rather unattractive entirely.
Please consult your history. I don't even necessarily disagree with your view on China. But it's not made better by essentially setting up a straw man, by suggesting the United States "as a democracy" is somehow especially immune to the same sorts of vicissitudes of history that may inflict themselves on China.
The more I read about American history, the less I think "democracy" really had anything to do with America's success. It does make for nice mythology though. Perhaps it is an accompanying comfort that history is highly contingent, and that it really could have worked out in a myriad set of ways.