Scholars over the last few decades dispute this, saying that immense advances occurred during these "dark times", in mining related technology in particular during the middle ages. Also clockworks, and much more. The Romans were great implementers but not renowned for innovation. As with so much else, the truth probably lies inbetween.
Lens grinding and gear-work indeed were important advances during the so-called "dark ages". There were also agricultural and metallurgy improvements. Whether such would happen faster or slower under original Roman rule is hard to say.
The end of slavery and birth of proto-democracies in Europe is arguably what propelled the industrial revolution. People didn't have to tolerate their lowly lot in life as much. When human labor got more expensive, machines proved their mettle. That increased the rate of mechanical progress until it was self-sustaining.
However, one can't date the beginning of the end of slavery even in England proper to any earlier than 1772. The advances that underpinned the Industrial Revolution were in place by that time, including the Newcomen steam engine - 1712. It's true that only ten percent at most of England's population were slaves in the Dark to Middle Ages but the rest were Villians/Villeins (serfs), tied to land.
It was actually the simultaneous agricultural revolution that made labor cheap because it made vegetables cheap. This revolution had little to do with machinery, with the exception of Tull's horse-drawn seed drill. (Admittedly, a big exception.)
I'm not aware of any kind of lens or gear technology being developed during the dark ages. Are you sure you aren't thinking of the Renaissance? The dark ages are called that because no one was educated and no one wrote anything down. There's very little recorded history during that time, which is where the name comes from.
Concave mirrors to correct poor eyesight were available in Ancient Rome, if you were rich enough (there's record of Nero using one at the forum, IIRC.) Button lenses (single small lenses) for embroidery, etc, perhaps as well. Literacy existed during the dark ages, largely in monasteries. Scotland esp, plus the Babylonian Empire (Late Roman Empire) persisted. Records were more scarce, but exist. Gears go back to ancient Greece, we now know.
https://en.wikipedia.org/wiki/Antikythera_mechanism
Pointing to things in ancient Rome or Greece isn't helping your argument, because many of those things were lost in the dark ages. They had toilets in Rome. They didn't have toilets in the dark ages. They built concrete-domed buildings in ancient Rome. They didn't build anything like that in the dark ages, or for a very long time afterwards (not until modern times; concrete had to be re-invented).
Literacy existed in monasteries, yes, but that was pretty much the only place it existed then!! Ancient Romans, by contrast, were mostly literate in the general population.
Why are people so eager to whitewash the dark ages and the loss of literacy and technology that happened there with the fall of Rome? I really wonder if a lot of it is coming from the Catholic Church; they've tried pushing a narrative that the dark ages were really a great time because everyone in Europe was Catholic.
