Nobody ever says: "the language is improving." In narratives of linguistic change the past is always pristine, the present a time of decadence, and the future a time when language will undoubtedly be worse.

What makes a particular time in the past especially prime to be the object of linguistic adulation? Maybe it's prescriptivism within that era itself that's to blame? So then the 18th century would be the ideal locus for this sort of idealization... or the 17th if you were French.