But the challenges are great. And here I want to respond specifically to some of the points that D&T make:
1. There are no speakers
True. Or almost true. That is, I can't vouch for the world's population, but I at least know of a few individuals that evidence high levels of communicative fluency in Latin, and a few in Classical and/or Koine Greek.
But more importantly, it doesn't matter. We're aiming to learn a language as it was, not as it evolves, and so our practices will not be seeking to recreate a living language in the sense of the neogenesis of an evolving language. We don't want the next generation of Koine speakers to be creative in the sense of linguistic evolution.
It's possible to learn to use communicatively languages that are extinct or close to extinction. Attempts to revitalise almost-dead indigenous languages, as well as revivalist efforts in classical languages, show that this is the case. So it's not a pragmatic obstacle.
2. Lack of creativity.
D&T point to the fact that Biblical Hebrew and Greek are closed corpora of texts. There are now no new instances of Biblical languages creating new content. This is true. But I also don't think it's entirely relevant. Hebrew, one might argue, is a slightly different case, but I'll leave that aside for the moment. Greek, surely, must be read against the background of Koine/Hellenistic literature, for which an even more expansive corpus exists. And that corpus has a critical mass – it gives us a significantly large enough picture of Greek as used in literature of varying styles to form a substantive communicative language (compare with Tolkein's languages, which have not supplied a vast enough corpus so that attempts to expand the language and literature are stymied).
True, every generation of a new statement in Koine Greek isn't authentic. It is an academic fabrication. But that doesn't make it value-less. If it remains consistent with the grammar of the Koine corpus, it's Koine.
The pedagogical argument is this: focusing on one's primary corpus: canonical literature, isn't as efficient or productive as pursuing a communicative approach that includes academic fabrication. Sure, pronunciation is disputed, but we can live with that. Sure, grammatical features of the language are contested. We can live with that. Sure, maybe no 1st century Koine speaker ever uttered what I might utter. We can live with that. We can live with that because a communicative approach will better allow a student to acquire an active proficiency in the language and more efficiently equip them to read the ancient literature in its original language.
3. The appeal to modern Hebrew and Greek.
I think this is a loaded issue, and I'm prepared to cop some flak on it. There are sizeable differences between the modern and ancient versions of these languages. My opinion is that those differences are marked enough that a student should not conflate them. I'm not opposed to using modern pronunciations, but I am opposed to those, particularly regarding Greek, who want to smooth over those changes in the language and say, “Why don't you just learn modern Greek – Greek's been alive all these years!”. Historical continuity does not guarantee, and cannot be used to presuppose, linguistic similitude. What most of us are interested in is the language as it was used in its historical settings around our chosen literature. I do think there are benefits in learning the modern versions of these languages, but I consider those benefits marginal as compared to acquiring a communicative competency in a version (even an academic reconstruction) of the ancient language. Learning modern Greek or Hebrew brings one into a contemporary language community, still evolving, and evolving further away from the fixed historical literary corpora. There's nothing intrinsically wrong about that, but let's be honest about it.
4. There's no such thing (as 'conversational' Biblical Greek/Hebrew)
Again, true, but I think not persuasive. There is no such thing as 'conversational' biblical languages available to us. Yet there exists enough of a corpus for us to know and use the language conversationally. Sure, there's a distinction, but if we're not aiming for conversational classical languages, but to use classical languages conversationally (if you follow my thinking), then do we really have a problem?
To summarise my argument (and I concede it's more a position than an argument that I have put forward):
Language acquisition differs from learning about language.
Students of ancient texts are better served by acquiring their respective languages.
Students of classical languages reinforce their acquisition best by conversing about their target language in their target language.
Communicative approaches are better equipped than Grammar-Translation methods to promote acquisition rather than learning.