As you say, there's definitely a Whiggish flavor here. Specifically, the question is: how do you measure decline? If your goal is to get from Galen to Galileo, then you have a framework within which to measure progress / regression. But if you don't assume that the only things that matter are the things that led from classical scholarship to modern science, and instead ask how changes in technology affected the lived experience of people lives in the medieval west, you'll get a somewhat different answer.

In terms of the continuity of classical learning, the west does indeed get rather marginalized in the early middle ages. Education in philosophy in the Roman world was incredibly expensive, but it paid off for those who could afford it because it gave a rich young man (mostly, but also a few women) the opportunity to make friends with peers from across the Roman empire. These relationships would last for life, much like the networking that elite modern schools offer (ie, go to Harvard because the friends you make will help you get ahead). And education taught you not only who to know, but how to interact with them; an education in philosophy included the necessary manners and mannerisms to succeed in elite Roman circles.

By the end of the fifth century in western Europe, this isn't how the imperial court worked any more. Emperors were being made and unmade by military strong men, and the ability to control your temper, deliver speeches in the style of Aeneas, and stay in touch with school buddies who were running their own corners of the empire wasn't very important (though it wasn't completely dead yet, either). Instead, status was tied up in participating in the culture of the army, which valued a new kind of masculinity that was all about bling, fur, and weapons. In this context, education was just expensive, and most people stopped bothering with it.

In the east, the story is much more complicated, and there was also a period of educational collapse in the eastern empire. But if the Byzantines slacked off, the new Muslim empires (which modeled themselves on the classical world in so many ways) stepped up and kept things going. Islamic scholars not only read and copied ancient texts; they expanded, debated, and reworked them into new centres of learning that had no western European counterparts. Charlemagne's 'Carolingian Renaissance' was a pale imitation of the Abbasid status quo.

And eventually this education came back to the west, taking root in Euopean universities in the 12th century and, by a road that wasn't as straight or obvious as any history of science survey might suggest, ending up here.

So in that sense, there's a gap. Education collapses in the west, and it's replaced with different social values that care most about martial virtues, not books. If we measure progress by education centres and learning, western Europe has a dark age.

But that doesn't mean there wasn't progress during the early middle ages in Europe. There was, and in ways that were more directly useful for the social values of the people experiencing that progress.

The early middle ages saw something of a revolution in iron technology. Steel production improved, and the quality of everyday tools jumped far above the average available during the height of the Roman empire. Knives were sharper, and they needed to be resharpened less frequently, in the 8th century than in the 1st.

The heavy plough was developed and refined, perhaps as early as the 7th century. This allowed more ground to be cultivated, increasing agricultural yield.

New loom technology was developed, leading to an exploding textile industry that helped launch England on a long journey toward the industrial revolution and global capitalism.

The Vikings built a new kind of ship that could sail to America.

So what's progress? If it's book learning, the west dropped the ball. But if your society values fighting, farming, and trade, the early middle ages were full of innovations. When we say 'scientific progress,' we tend to snub the practical side of technology and assume that only theoretical discussions count. That's a perfectly valid position to take if that's what we value, but the consequence is that we can easily forget that things which don't meet that particular definition of progress are easily overlooked, even if they had profound consequences on important things like standards of living.

So if our goal is to explain the origin of modern science, the west is indeed cut off. But we must remain open to other narratives based on other goals if we want to get the whole picture.