premise: Johnson had no trouble finding fairly exact counterparts in his London of the eighteenth century for many of the blights of Juvenal's ancient Rome, but there was really nothing to match the horror of the amateur poetry reading in Trajan's time; the closest analogy Johnson could come up with, an elegant one under the circumstances, is his  female atheist. 
hypothesis: Nothing was seen to match the horror of the amateur poetry reading in Trajan's time.