Read my comment. I specifically mentioned Britannia. The Franks (i.e. Merovingian), had less than 20% of the former Roman Empire in control. About 18%. The rest of Europe was extremely fragmented, and territories changing hands often (and always through war).
Mentioning that there were plenty of building and construction happening in some part, doesn't negate that the rest (80%) was being ravaged and in continuous decline to the point of whole populations being displaced.
Only the Eastern Roman Empire (aka. Byzantine empire), had a unified government structure, yet it was struggling with the Slavic invasions/incursions from the north, and the wars with the Sassanid empire and the succeeding caliphates.
They did attempt to re-unify the empire with the Justinian Restoration (with Belisarious being the main general re-conquering vast territories), but that didn't last long either. As the economy was lagging the region was ravaged by the Black Plague, where it is estimated that 25–50 million people in two centuries of recurrence died, equivalent to 13–26% of the world's population at the time
https://en.wikipedia.org/wiki/Plague_of_Justinian
To me, late Antiquity end, and the true dark ages begin after the death Justinian, and the decline of the Eastern Roman Empire to the point that it couldn't defend or keep its territories from the Slavic invasions.
