Nope not a professor, sorry, though all my hithero academic training has been in Classics.
It’s interesting you point out propaganda, and it kind of is nowadays, but I think the myth of conservatism partially arose out of indifference! If you look at manuals of “modern” Greek from the 10’s and 20’s you’ll see very little ancient there. Classically educated Europeans were keen to speak Greek and Greeks in the diaspora were presenting as Classicised a version of Greek as they could get away with. There’s a lot of interesting intellectual history here actually. Suffice to say when these people went to Greece they couldn’t actually speak with actual Greeks lol.
The problem is the sense of “conservatism”, conservative in what? there are several facets to each language (phonology, morphology, syntax, lexicon - semantics thereof, etc) and no language is going to be equally conservative across ALL. Greek is impressively conservative in the sense that it hasn’t broken up into differing languages - yet even so pre-modern dialects are quite different! Tsakonion, living Doric, has θ as Σ and η pronounced like ε (contra modern Greek ι) and υ as ου where standard as ι again. Romeika in Turkey has an infinitive, Pontic is morphologically more complex and hasn’t been as iotacised etc etc. Even so we haven’t seen the level of break up which gave rise to the Romance languages.
“but I always thought it rather strange that Greek would change so slowly considering Greece’s total lack of geographic isolation and all the invasions, trade networks, etc.”
This is key! Honestly most Classicists are bloody clueless of the early middle ages onwards, the foreign element in Greece was very…prominent, but are ignored by wishy washy thinking.
So if you briefly sketch out changes from ancient to modern. You have phonology (which I shan’t cover, well known), the loss of dative (which means no verbs with dative etc), the loss of several third declensional noun classes save very literary Greek (but again Pontic has retained some), the lack of participles (one of the defining features in ancient Greek syntax, we only have a few left and they’re not productive), athematic verbs, infinitives, optative mood, subjunctive mood (να + verb isn’t really a proper subjunctive), the particles, several clausal markers have been list or assimilated etc I could go on but I’m actually bored of this list. That is a huge ass list and the difference between Modern and Elizabethan English isn’t a fraction of that (which is largely a productive of orthography outside of the vowel shifting).
A lot of the so called conservatism, then, is a product of everyone learning ancient Greek and propaganda.
There are several good sources. The best for post-classical Greek is definitely Geoff Horrock’s “A History of Greek and Its Speakers” which has, thankfully, finally came out in paperback form. As an undergraduate I had to fight to get hold of this book, it’s the best book in any language on such a topic and translations will apear most surely. There’s been a Greek one for a few years and I’m sure more will follow. Other than that, I always recommend the Blackwell Companion to Ancient Greek Language here because it has some bloody good articles, especially the one on phonology (though it pales in comparison to its Latin cousin, alas).
Btw it’s interesting you mention Icelandic. I’ve always thought it was hyper-conservative though, but I’ve recently been told by a friend (who is a Germanic Philologist) that actually this conservatism is recent, that there was a deliberate purging of the language a while back and thus it’s not natural conservatism. I had NO idea this could be successfully done. Honestly, clawing my way through the Volsung saga very slowly…I’m envious that they get such a head start.
Sorry we’ve sort of veered off topic, but to bring us back may I just say that these changes were precisely the reason why Greeks studied their own language and got similar benefits to Romans learning Greek.