On p 124 of the second edition (1974) of Scribes & Scholars. A Guide to the Transmission of Greek & Latin Literature, LD Reynolds and NG Wilson state what I had begun to strongly suspect after I had begun, roused to indignation by Stephen Greenblatt, to study the career of Poggio Bracciolini: after pages of praise for Poggio and his fellow 15th-century Italian humanists for their many discoveries of ancient Latin texts, Reynolds and Wilson add:
"The humanists also had a capacity for losing manuscripts. Once they had carefully copied a text, they were liable to have little interest in the manuscript which had preserved it."
Would we have more Classical Latin manuscripts written before the 15th century if Poggio had gone into another line of work -- say, painting? It's probably impossible to say. We value manuscripts, especially old manuscripts, much more highly today, and, all other things being equal, we tend to value them more the older they are. It causes us great pain, in case after case, reading about the discoveries of the humanists, to read that familiar description of a manuscript discovered by Poggio or one of his colleagues: "now lost." "deperditus"
Reynolds and Wilson go on, ibid:
"In the sixteenth century the situation was worse; many fine codices went along the one-way road to the printing press."
That is to say: a Classical Latin text was printed, and the manuscript or manuscripts which had been used to make the printed version were lost. In some cases, the printed editions are all that is left of the texts.
Obviously, this unfortunate process wasn't universal, or else we would have no manuscripts of the Latin Classics today. Angelo Poliziano, another 15th-century humanist, sounds much more modern than many of his time in his emphasis upon preserving and consulting old manuscripts, the older the better (Reynolds and Wilson, 127-129. They even say that the way Poliziano valued age in manuscripts was "too sweeping"). Poliziano was neither the first to recognize the value of old manuscripts; nor did his emphatic defense of their value change the practices of Classical scholars all at once. He planted the seed of the idea, as did others before and after him. Gradually it took root.
There are a great many 15th-century manuscripts of the Latin Classics still existing today, probably many more than those produced in any other century. Is this because there was an explosion of interest in these ancient texts, or because the idea was gradually taking hold that it was good to preserve manuscripts, or simply because the 15th century is the most recent one before printed books replaced manuscripts? I'm sure that all three factors played a role; I'm not going to guess how much of a role was played by each.
Like the 15th century, the 9th is represented by far more Classical Latin manuscripts than any previous century. Charlemagne saw to that with his immense program of revival of education. The total number of Latin manuscripts made before the 9th century, not just Classical but also Christian, mostly Christian, all noted in the Codices Latini Antiquiores,
the great work of EA Lowe and his followers after his death, comes to about 2000. In the 9th century alone there are far many more manuscripts than 2000. How many, exactly? We don't know, because there are so many that so far no-one has found it worth the tremendous effort of seeking them all out and listing them all. Suddenly, in the 9th century, Latin manuscripts are no longer nearly as rare. Counting just 9th-century Classical manuscripts, do we currently possess 2000 of them? I don't know. I don't know whether anyone knows for sure. Might well be.
And yet, all of the 9th-century Classical manuscripts are copies of older manuscripts. And, just like in the 15th-century, the processes of discovery and preservation were partly also processes of destruction: once those older manuscripts had been copied in the 9th century, many of them tended to be lost. The difference is that we don't know nearly as many of the details of these 9th-century losses, because we have far fewer letters and other items which would inform us from the 9th century, than from the 15th. We have, however, recovered some of those pre-9th-century manuscripts which were written over or made into book covers.
The 15th and 16th centuries was the great age of re-discovery of the Latin Classics. It was a river, and what has been re-discovered since then has been a trickle. But however many more Classical texts may still be discovered, there remains very much to do in investigating the processes of textual transmission. Between greater historical understanding, continued technological progress and the blessed dogged persistence of humanity, I remain optimistic that great discoveries of Classical Latin are still to come.