Maybe the ancient civilization will not collapse and we would have smartphones and satellites around year 1000 AD.
Also the 'dark ages' weren't really as dark as some might think. Arguably most of the decline was a result of climate change and the plague, which lead to a collapse of urban populations, which were the centers of literacy and education. However technological progress didn't really stop, there were significant improvements in metallurgy and agriculture and by the high middle ages Europeans were much more 'advanced' in almost every area compared to ~400 AD.
The one thing ancient empires were exceptionally good was concentrating resources, e.g. no state was able to field a standing army in the hundreds of thousands and no city even came close to the population Rome had until the 19th century but that was due to political reasons (shipping tons of grain half across the (known) world to fed a massive unproductive urban population just to shore up some political support simply wasn't viable anymore).
In the east, in Anatolia, the Roman civilization lived on with quite a bit more vibrancy and power than people often recognize.
But in places like Britain it's pretty night and day: early 5th century still a predominantly Christian Roman province speaking Latin and Brythonic, by the end a predominantly Saxon speaking mostly non-Christian population with drastically reduced economic activity and wealth.
You can't raise taxes when half or more of the population is gone. As per WP: "Some historians believe the first plague pandemic was one of the deadliest pandemics in history, resulting in the deaths of an estimated 15–100 million people during two centuries of recurrence, a death toll equivalent to 25–60% of Europe's population at the time of the first outbreak"
And this is after a couple generations of political turmoil, other plagues, and invasions.
Also we don't really know Christian Britain was before the Saxon conquests, it's not unlikely that most British people outside of urban settlements were still pagan and only superficially Christianized. We also don't really know did the British people become culturally Saxon. According what was written down by the surviving members of the Romano-British elite it was a result of genocidal conquest. However the predominant view of modern historians seems to be that the process was much more peaceful and more akin to the Arabization of the Middle East and North Africa (i.e. the conquerors established a dual class system which incentivized the local population to adopt the Saxon language and culture because of the economic and social advantages. Also, unless the Saxons were significantly more brutal, oppressive and numerous than the Franks, Goths and other invaders this would imply that the local Britons weren't actually thoroughly Christianized since Christian populations were usually able to resist this type of Cultural conversion).
In regards to the comparison between Franks and Goths and Saxons, it's not really the same. In those cases they took over the existing Roman state and continued its administration, leaving the institiutions of civil society much the same and even adopting Latin as a language and Roman cultural practices for themselves within a couple generations.
By the time the Saxons arrived en masse in what is now England the existing Roman administration had pretty much collapsed. That seems to be the archaeological evidence in my recent reading: drastic declines in the economic vibrancy of British communities. Things like amphorae and other mediterannean trade goods vanishing. Rich people burying their wealth to hide it from invaders. And, yeah, the stories told by Gildas & St Patrick, our only real contemporary accounts from this general period, seem to agree with this: decline of local British power, retreating into small cloistered communities, Saxon (and other Germanic) raiders wea continual pest even in Roman times, yes, but after the departure of the Roman administration it becoming drastically more aggrivated, the locals were incapable of defending themselves, and the British turned to Germanic mercenaries as a defense (a common Roman tactic). Which predictably backfired.
I've read two books commenting on this period of British history recently, and both disagree remarkably on the degree of cultural and physical replacement. But on the whole the genetic and archaeological and linguistic evidence seems to me to support the statements by Gildas and St Patrick: the British were on the whole replaced & expelled (to Wales, Cornwall or Brittany), or exterminated. This is reflected in part by the fact that only a few Saxon settlements carried forward on top of existing Romano-British settlements, and most adopted new names entirely. Even some prominent ones like London were left pretty much uninhabited for generations before being re-settled later.
There is evidence that there was at least some degree of peaceful coexistence between Saxons and local British at least in the early periods of settlement. There are burial sites where both genetically British and German (or mixed) people were buried together, with identical grave goods and seemingly with no distinction. This would indicate that at least some local people adopted Saxon customs and language and were accepted as equal members of their society. However this is not exactly conclusive, there are also many segregated burial sites so the type of interaction between the local Britons and the German invaders/settlers varied by area and time period, it took them well over 200 years to almost fully conquer the territory of modern England.
Gildas and St Patrick were members of the Romano-British landowning elite which was displaced by the Saxons, I'm trying to dispute their authenticity but their experience might not have been representative of the majority of local people (who were never fully romanized or integrated into the roman society). It would not be completely surprising if local Britons decided that they would be better off by 'switching sides' and adopting Saxon/German culture. Of course we don't know how widespread and voluntary the process could had been, however according to genetic evidence it must have occurred to some extent.
But, yeah, unlikely.
I think most people would consider that a “dark age” or “collapse”.
However outside of that period it's not that straightforward. The Roman economy was stagnant or declining well before the Western empire officially collapsed. According to some historians growth in much of the Mediterranean region stopped with the Roman conquests at the end of the Hellenistic period. But even if we assume that ~200 A.D. was the peak can we really say that there was more technological innovation and economic growth between the 3rd - 5th centuries than between the 8th and 10th centuries? Probably the latter..
And it also varied significantly depending on the area we're talking about. Italy probably took the longest to recover but it's economy was massively inflated due to wealth transfers during the Roman period. However the center of economic and political power in Europe has shifted to the other side of the alps France and Germany had nearly the double the population in 1000 AD compared to their Roman peaks. And some places like mainland Greece arguably never did until the 19th century (it was probably was past it's peak during the Roman period as well, though. A similar process occurred in the eastern Mediterranean where the center of power shifted from the coast to the inland areas (like the area modern day Iraq).
And we're only taking about absolute growth here, per capita productivity probably wasn't as much affected and most of the decline in international trade (which eventually recovered) was balanced out by higher availability of land (per capita) and improvements in agricultural techniques and tools.
If we're talking about technological growth, well again in some areas like architecture the decline was severe and near total (most communities couldn't afford to maintain the buildings they already had). In other areas the reverse might had happened, based on archeological evidence the average quality of metal tools and weapons in Western Europe was already much better in the 7-8th centuries compared to what the Romans ever had. The heavy plow was introduced in agriculture, water mills continued to be built. Literacy rates fell, however that was to a large degree an outcome of the collapse of urban populations. However the link between economic and technological progress and education in the ancient world wasn't all that clear.
Once Rome fell, the educational system outside the Church disappeared entirely for almost a millennium and the Church remained the only institution that carried on the tradition of reading and writing in the West.
This must have had some impact on the tempo of overall technological progress. Just like the fact that the successor states, unable to concentrate resources as much, couldn't start big public works (channels, aqueducts) which used to require a lot of highly qualified people.
I don’t know if the facts actually back that up.
If the intelligentsia are focused on climbing fruitless political ladders steeped in theology versus scientific endeavors, you have less progress overall.