> There is _no_ difference in standard orthography, because yat reflexes have nothing to do with national boundaries

But there is, though, e.g. "dolijevati" vs "dolivati". And sure, standard Serbian/Montenegrin allows the former as well, but the latter is not valid in standard Croatian orthography AFAIK. That this doesn't map neatly to national borders is irrelevant.

If Australian English is so drastically different that Australians "won't speak English in 2-3 generations" if their orthography is changed to reflect how they speak, that would indicate that their current orthography is highly divergent from the actual spoken language, which is a problem in its own right. But I don't believe that this is correct - Australian English content (even for domestic consumption, thus no code switching) is still very much accessible to British and American English speakers, so any orthography that would reflect the phonological differences would be just as accessible.