> Serbian, Croatian and Bosnian
are based on the exact same Štokavian dialect, ignoring Kajkavian, Čajkavian, Čakavian and Torlakian dialects. There is _no_ difference in standard orthography, because yat reflexes have nothing to do with national boundaries. Plenty of Serbs speak Ijekavian, for example. Here is a dialect map: https://www.reddit.com/media?url=https%3A%2F%2Fi.redd.it%2Fc...
Your example is literally arguing that Australian English should have the same _phonetic_ orthography, even. But Australian English must have the same orthography or else Australia will no longer speak English in 2-3 generations. The difference between Australian and American English is far larger than between modern varieties of naš jezik. Australians code switches talking to foreigners while Serbs and Croats do not.
> 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.
By tautology, if you split the language, you split the language. Different groups will exhibit divergent evolution.
> current orthography is highly divergent from the actual spoken language, which is a problem in its own right
The orthography is no more divergent to an Australians speech as to an American's speech, let alone a Londoner or Oxfordian. But why would it be a problem?
