Some of this would obviously be lost in a civilizational decline regardless of existence of extra-monastic science. But I mentioned books of Livy histories which would be preserved if people were actually interested in lay history of Rome, their supposed quintessential state. Today we cannot figure out many facets of Roman Republican system in like 1st century BCE and before. If Classical-style philology survived on a serious scale, we would get the second part of Aristotle's Poetics and more of the classic Greek tragedies and poetry. We have only scraps of Hellenistic philosophy (Stoic and even moreso Epicurean, Sceptic), and mostly only the Latin imitators, because again they stopped reading Greek and had to avoid these "suspicious" worldviews. The books on these topics, if you actually read them, have to largely rely on connecting scraps and conjecture. We cannot really say we "actually got their most esoteric and intellectually interesting stuff" with any certainty.
I think the point of monasteries monoculture stands, but it's really secondary. My main point is that cultural decline cannot be explained and nuanced away. Say, we wouldn't get the Arabic and Byzantine retransmission, and got only the things preserved in the West in 700 CE, would you also be so content?
> which would be preserved if people were actually interested
And this is the non-obvious point that the first part of OP disagreed with. Preserving and copying texts on any sort of scale without anything like a printing press was hard, specialized work. And there were few or no institutions explicitly devoted to that task - to the best of our knowledge, anyway - prior to the Christian monasteries. The Musaion and Library of Alexandria is described as exceptional simply because it did engage in some primitive version of that work, and even that was not considered important enough to be kept functional for more than a few centuries.
> copying texts on any sort of scale without anything like a printing press was hard, specialized work
Not disputing that it was an expensive luxury thing, but let's not take it to the extreme that the ancients were helpless in that regard. Clearly they were able to preserve a number of titles throughout centuries, there were many books commonly read and cited. (Until they disappeared toward the late first millennium.)
If some piece was really core and central to them, they generally managed not to lose it, just because of the need (again, not some abstract desire for preservation). If they stopped, it means that either they collectively lost interest in some swaths of intellectual activity, or lacked the means because of an economic and civilizational slump. In fact both those factors were likely at play here. The transition from scrolls to codices, that you are probably alluding to, was a force multiplier for this process.
Some of this would obviously be lost in a civilizational decline regardless of existence of extra-monastic science. But I mentioned books of Livy histories which would be preserved if people were actually interested in lay history of Rome, their supposed quintessential state. Today we cannot figure out many facets of Roman Republican system in like 1st century BCE and before. If Classical-style philology survived on a serious scale, we would get the second part of Aristotle's Poetics and more of the classic Greek tragedies and poetry. We have only scraps of Hellenistic philosophy (Stoic and even moreso Epicurean, Sceptic), and mostly only the Latin imitators, because again they stopped reading Greek and had to avoid these "suspicious" worldviews. The books on these topics, if you actually read them, have to largely rely on connecting scraps and conjecture. We cannot really say we "actually got their most esoteric and intellectually interesting stuff" with any certainty.
I think the point of monasteries monoculture stands, but it's really secondary. My main point is that cultural decline cannot be explained and nuanced away. Say, we wouldn't get the Arabic and Byzantine retransmission, and got only the things preserved in the West in 700 CE, would you also be so content?