Old Persian was an inflected language like Avestan and Sanskrit. Farsi is so, no longer. Apart from the vocabulary and the Semitisms (Aramaic then Arabic), it is pretty-much a pidgin of the ancestral tongue. As of last month Kevin van Bladel is explaining it all - for free.
The pidginism thesis, for van Bladel [POST-READ 10/10 -semicreole
], came from a chapter in John McWhorter, Language Interrupted (2007). That was before McWhorter's late disgrace. The chapter in 2007 was impressionistic, but still insightful, so van Bladel is now doing justice to a topic which McWhorter - I fear - cannot anymore do himself.
Van Bladel sees the most drastic pidginification in the Achaemenid era. Old Persian was still inflected for Darius I, at least for Darius' monuments. Darius III presided over a language well on its way to the Middle Persian of the Sasanians which, in turn, isn't too far from late-mediaeval Farsi.
This process is what the book essays to sketch. For van Bladel, pots come with people: one people brings language - the pots - and other peoples must react based on how the pots are presented. Greeks, Indians, and Armenians all had to learn Persian; perhaps less hard for them. But so did Aramaeans and Elamites. First, these two languages simply weren't/aren't IndoEuropean, nor are they relatives to each other. More importantly these two were the preponderant populations at the imperial core, where "Javonia" was, like Egypt and Caria, peripheral.
Looks fun. Will read.