> E.g. a century after the Romans left Britain, it would be fairly obvious to everyone that whoever built the aqueducts, villas, fortresses etc had vastly superior technology.

Yes, but

1. Britain was where there is the best case for a serious regression. 2. Building those systems was also a matter of imperial priorities and imperial centralisation. Smaller kingdoms did not need it.

> And much of the literacy was aimed at preserving what knowledge had survived from the classical period

Much was, and Aristotle was taken as far too much of an authority. There were probably not many advances in science during the early middle ages, but there were in high and late medieval. Even in the early middle ages there were advances in architecture and agriculture and some amazing art produced.