I read in a documentary about Roman Britain that the withdrawal of the Roman Empire from Britain set back the clock back quite a bit. In fact, immediately pre-roman Britain seemed to be a bit ahead of post-roman Britain (5th-6th centuries). That got me thinking. How far back did the collapse of the Roman Empire in the west set back civilization? And how quickly did they recover?
I know Spain recovered quickly when the Arabs invaded, and then the Byzantine Empire probably also helped with rebuilding in some areas, but I'm not sure about other areas of the empire in the west.
Britain and Italy were two special cases of regression, it's not possible to guesstimate how things would have continued to evolve if the Roman Empire hadn't withdrawn from the West and how that compares to what happened in real life, but it's worth noting that each region was different.
France, the Low Countries, and Western Germany relatively quickly came to be dominated by the Franks, who had sophisticated and powerful states. There was certainly change from the Roman Empire, but Gallo-Romans and Franks co-oexisted in the Frankish state(s). I would rather have been born in France in 500 AD rather than 400 AD for sure.
The Visigoths had a relatively stable kingdom in the Iberian peninsula for over 200 years, it wasn't as stable or prosperous as their Frankish neighbours, but it was a functional state with an impressive culture. The man known as the "last scholar of the ancient world", Isidore, Archbishop of Seville, was a key player in this kingdom. Romans and Visigoths also coexisted peacefully in the Visigothic Kingdom.
The Vandals in North Africa were not as high functioning. North African Trinitarian Christianity had been beset by significant internal challenges for over a century when the Vandals showed up, and in an unfortunate accident of history, the non-Trinitarian Vandals were the most aggressive of the Germanic peoples in promoting their version of Christianity. This persecution led to social division and was a factor in allowing the Eastern Romans to retake North Africa relatively easily about 100 years after the Western Romans lost North Africa. The contentious history of Christianity in North Africa is, IMO, the principal reason why Christianity vanished so quickly when the Muslims invaded, in contrast to Egypt, where Christians remained the majority for centuries after the Muslim conquest, and still constitute a significant minority in Egypt.
Italy functioned relatively well and with a great degree of continuity under the Ostrogoths, they continued to recognize the Eastern Roman Empire in theory, and kept many Roman institutions. Ironically Roman Italy was truly destroyed when the Eastern Romans conquered Italy in the Gothic Wars. Ultimately the Lombards invaded and were much less interested in equality and coexistence with Romans, but the Roman society and economy had basically been destroyed by decades of war at that point.
England was another special case. The Anglo-Saxon invasion and settlements were much less organized, there was very little continuity, the economy was disrupted, Christianity retreated into the hills, and there was significant population decline. While historians don't like to use terms like "Dark Ages" to describe post Roman Europe, it is pretty accurate to describe the situation in England for large parts of the 5th and 6th centuries.
The Welsh maintained a consciously post Roman identity and held onto their Christianity, but there were impacted as well by interactions with their new Anglo Saxon neighbours.