> 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.
I chose Roman Britain because I live in a city (Deva) which was once meant to be the capital of the whole province, where the evidence of lost glories would have been glaringly apparent to everybody for a long time, even though the settlement was still major in contemporary terms.
More generally, some technology was lost everywhere (well, in the Western world anyway): nobody knew how to make waterproof concrete again until a manuscript reappeared in the fifteenth century.
Roman Britain is just one example, but it does disprove the general thesis that people always think they are the pinnacle of civilisation – and it's by far from the only example, of course. For much of the next thousand years (and beyond) Classical Rome, and later, Ancient Greece were seen as a lost golden age, something to learn from and aspire to (and adapt to religious dogma in a fallen world which was going to end fairly shortly anyway…)
Of course they had their fair share of brilliant people and they made significant advances and it's facile to disparage them ("Dark ages") but it does seem like a very different mental view of the world.