I note that I do have a reasonable grasp of science since I am trained in science and I work in science

. However, I was using the words "doomed" and "theory" in a popular not scientific manner that befits a chat site. Apologies, I did not intend this to be taken the wrong way and linked to other examples like the Bison, as I don't think their decline has absolutely anything in common with OBP. However, a couple of more points for consideration:
All threatened species listings are made on the basis of looking at past, present trends and predicting the potential for future extinction, with and without intervention (using both quantitative and qualititative criteria). It's not exact but it still applies "good" evidence-based science. Each category is based on the likelihood of extinction over certain timeframes eg. critically endangered for OBP means that the scientists on the national Threatened Species Scientific Committee determined in consultation with relevant experts that it is "facing an extremely high risk of extinction in the wild in the immediate future".
The point that the renowned threatened species scientist was making to me about OBP was that he thinks that small parrots like OBH are not generally well adapted to long sea crossings, unlike other migratory birds. I've seen no evidence to suggest either way whether OBPs were already decreasing prior to European settlement (you suggest they weren't but my source thinks it's likely). However, as I mentioned in my original e-mail I agree there is no doubt that anthropogenic habitat destruction and other threats have been extremely detrimental to their survival. Also, just to be clear, I would not for a second suggest that such a species is not worth saving. I think we should make whatever efforts we can to stop a species from going extinct, no matter what it is (although sometimes the investment comes too late and unfortunately is scaled back if there is no success e.g. recent/late efforts Xmas Island Pippistrelle).
Regarding the Blue-winged Parrot, I note it is secure on the mainland but Endangered in Tasmania, so in fact, at the risk of inciting further criticism for bringing the sea level rise issue up

....this e.g. could further support the point that the Bass Strait sea crossing is becoming too much for some of these birds too (but that it has had more success at surviving as a species because enough birds occur as far north as Queensland and don't make the sea crossing).....