The western part of the Roman empire didn't die out until four hundred years later. The eastern part took another 1000 years to bite the dust.
I believe we are on the beginning of a long decline that may take centuries to unfold. It's sad, because it looked like things were on the up-and-up through the 50's. Some would even say things kept improving, at least technologically and in some ways culturally into 2000.
I think we're now talking about "Peak US". And no amount of flag waving or talk of taking back the country is going to stop this. The country has been taken and taken in by our liberal culture.
So I don't think there's going to be some dramatic apocalypse type moment in our lifetimes. Things are just going to get more restrictive, more annoying, more cumbersome, more unspeakable, more licentious, more illiterate, etc.
I think you are ignoring the acceleration of the rate of change brought on by technology.
The Roman world was not much different from the world of a thousand years before, or of a thousand years later. But look at the changes in the last two hundred years; or one hundred years. My Dad was born into a world that transported goods to market via steam engines and wagons drawn by draft animals. Electricity was a novelty. Most Americans lived without indoor plumbing and used gas or oil lights. Airplanes and automobiles were new and novel innovations. Radio as entertainment did not exist and TV did not become common until he was in his forties.
When I was a kid, the milk was delivered by a man in a white uniform with a peaked cap and a bow tie. We got our vegetables from a horse drawn wagon that came around the neighborhood; meat at the butcher shop and bread from the bakery (Where day-old bread was cheaper). There were no supermarkets or shopping malls. Our telephone was a party line for years and I was 18 the first time I saw a color TV. I was 47 years old when I first heard the term "Internet".
Our way of life is not just changing. The rate of change is accelerating exponentially. That is very scary for a lot of folks, especially my contemporary geezers.