> For most Greek speakers texts from this time are mostly legible, words like epiousios not withstanding.

Is this true? As someone who studied classical Greek, I can mostly understand NT Koine OK, but most of modern Greek goes right over my head. Conversely (anecdote alert) I watched a YouTube video where a Greek guy took ancient texts out onto the streets of Athens and asked people to make sense of them; most people giggled self-consciously and admitted that while they had studied them in school they were able to make very little headway in actual translations.

Edit: so my point is that, to my naïve eye, Koine seems much closer to Attic than to Modern, and hence I'd expect modern Greeks to struggle with it - but would be very interested in being proved wrong :)

In my own limited experience in school, I found texts from the first century AD much easier to comprehend than say the odyssey, or even classical texts.

But still it was an academic exercise, it required attention and some time, it was far easier than classical texts yet not something you can do in the street on the fly.

I may have to pull back from mostly legible to mostly decode-able. ;)

I only really know some classical texts and, for Koine, just the NT, but the structure of the NT is so much more straightforward to understand - it's basically this happened, then that happened, etc etc and there is none of the "decoding" that you often have to do with the classical stuff. But I am fascinated by how much of this is intrinsic to the language and how much is down to artifice on the part of the classical writers. In other words, the audience for the NT is ordinary folks whose first language in many (most?) cases is not even Greek, so the message has to be written as plainly as possible (and actually the prose is kind of pedestrian as a result IMO). But what's the situation with classical authors? Are they really reflecting the speech of the time or are they using a more complex and artificial form of language divorced from daily speech? I don't know - I think there must be an element of it, but otoh someone like Aristophanes is trying to make audiences laugh so the language has to be relatable and idiomatic you would think.

This, exactly. The Greek alphabet is much older than Koine Greek, of almost 500 years. While spoken language tends to change fast, written language tends to stick and doesn't easily change, for obvious reasons (just look at English or Tibetan). People won't change how they write some word just because they pronounce it differently from the past, because that would make the text much harder to read.

Also, /b/, /β/ and /v/ have the tendency to get swap with each other in lots of languages, and most people fail to distinguish them apart unless their language imposes a clear distinction among them. See for instance how Italian or English distinguish /b/ from /v/, while /b/, /β/ and /v/ are all basically the same thing for a Spanish speaker.