I skipped chapter 2 because it's covering similar ground as Mark Durie. Between chapters 1 and 3, one might explain why it was that Europeans thought that the Persians had degenerated under their Achaemenid shahs. To use the Middle Persian for xšāyaθiya.
Germans and, maybe, Russians and certainly Greeks and Latins could see that the Persian language changed 500-350 BC. I pick on those four because they retained, stubbornly, the inflexions of their own languages; against such pressures as applied to English and French. But the English and French orientalists were saying the same. Since Beowulf and Tolkien I suppose some of the English have inflexion-envy. Frenchmen might, also.
Value-judgements might also be brought at the empire filling its palaces with eunuchs and harems. Van Bladel, head-scratchingly, makes a point of refusing to judge. Okay. The Greeks sure diagnosed the Persians as an unsteady empire, and literally wrote the book on politics. Eventually the Greeks proved that thesis by destroying their empire. Difficult to dispute results.
Even before the disaster
of Alexander III's takeover, Persians already could see that the spoken common language was not that of Behistun anymore. This embarrassed some, who then tried to spell their monuments in the good old way. It appears they had no textbook for that; so such inscriptions ended up being... wrong. Like the Book of Mormon (in its first editions) did Tudor/KJV English wrong. If what you put out there isn't evolved but is just wrong, someone is going to point and laugh at you.