That is a pool of 120 million readers to produce the tiny fraction that would produce works worth being broadly read. Its even entirely likely that many lacked the resources to become prolific skilled writers read by others even if they had the talent there being a significant barrier to entry in those days.
Now we have a population of 7.6 billion of which 85% can read. This is a population of almost 6.5 billion readers.
If people are 1/10th as likely to produce good works then we are apt to have 5 times the number of good writers.
For there to be half as many good writers we have to believe that multimedia has extinguished 99% of the inclination to write.