I think forum are better SEOed than mailing lists, so old content should be more easily
reached for future reference, or at least I infer that from my search experience in the past (stackoverflow being the best example, though it’s not a classic forum of course).
than again, I for one prefer to interact by mail than through a site. It would be nice to have the best of the two worlds though.
Some questions that come to mind: what would be the purpose of the discourse site? would it coexist with the ml? would it be used sufficiently to justify it? to me the “other languages have adopted it” isn’t in itself a valid reason to consider it, maybe you can explain why you think would be nice or useful?