[evol262]

This is completely arbitrary. Both from the sense that, as you mentioned, there are a number of inflection points prior to that, and in "Greek-speaking" being distinctive at all.

If the classical Roman citizen weren't horrified at the organizational changes under Diocletian, or the difference in response between Adrianopole and Cannae, or moving the Imperial seat, or Romanized barbarians leading legionary armies (all of which were 100+ years before the fall of the west), they'd keep going.

The fact that the population in the East spoke Greek (and Syraic, and Aramaic, and a bunch of other languages) is a meaningless distinction. The West never truly had a formalized language anyway, at least among the citizens. The East had the same history, the same institutions, the same government