A lot of this stuff is just ship-of-Theseus for politics; institutions and power structures gradually break down and are replaced piecemeal with other ones, making it difficult to identify a cutoff date. Not only that, what are the boundaries of "our" "civilization" - what's not ours, and what's not civilization?
Trying to stay at a meta level, this discussion is almost intractable _because_ of the issue of definitions. It reminds me of the SCOTUS 1964 case for obscenity, where one justice notes "I'll know it when I see it" [1] You can look at, say, Western Europe after 700AD or so and see that things are not as good as they were 500 years before, but it's quite arguable which (if any) of those things constitutes a "decline" Charlemagne, after all, was crowned (among many other things) "Emperor of the Romans" [2]
In fact, even during its heyday, the Roman Empire defied defied definitions. Were you Latin? Roman? A citizen? These and other terms all meant different things as the civilization progressed. There was almost a determined effort to keep shifting definitions around such that the Roman Empire never died. In fact, IIRC, there's a 60 Minutes segment where they interview some Greek monks living on an island. The flag they're flying? The flag of the Byzantine Empire, of course. (Also, the word "byzantine" is mostly our own invention. They considered themselves thoroughly Roman)
Having said all of that, it's completely obvious to any outside observer that major changes occurred, many of which had a negative impact on the population. So something happened. Until one starts in with definitions of what exactly constitutes a valid OEM part for our Theseusian ship and what doesn't, folks are free to completely ignore the discussion, no matter how bad things get.
