That's true, the decline in literacy was significant, but this is to large degree an outcome of the collapse of urban populations, even during Roman times there

That's true, the decline in literacy was significant, but this is to large degree an outcome of the collapse of urban populations, even during Roman times there were relatively few literate people in the countryside. And "almost a millennium" is probably stretching it a bit, for example in Tuscany (and presumably other regions of Northern/Central Italy) most small towns or even villagers seemed to already have teachers by the 12th century and in richer cities like Florence at least the majority of boys attended a primary at least for some time (by the 14th century it's estimated that up to 30-35% of the people living in Florence could read, of course being one the richest and most developed cities in Western Europe this was probably an outlier, but then again even during the Roman times literacy probably varied significantly between Italy, the Greek cities in the east and places like Gaul or Britain).