Ummm no... Look at Lviv, Brasov or Cluj for example and remember those were the largest cities in their respective regions. Across Eastern Europe of the period the ethnic makeup of cities could often be quite different to the agricultural regions surrounding them. Another problem of dividing up these territories into 'rational' borders.
And that map doesn't show historic claims - Pressburg (Aka Bratislava) being the historical capital of Hungary, Prague being a majority German city until, what, the middle of the nineteenth Century. Given that that was a map representing 1913, that represents the same amount of time in passing as between now and the era of Second World War and Stalinism.
(Also note that this map doesn't even bother to distinguish between Croats, Serbs and Bosniaks, who all after all spoke effectively the same language)
Ah, you may not realize this but you are engaging in that classic American historiographical trope of arguing that all the world's problems might have been different
had only AMERICA taken the right course.
Of course, that's nonsense as is this argument. America had no way of enforcing Central European land boundaries - the boundaries that were 'fixed' by post-war treaties were often ad hoc justifications for land grabs that had already taken place (How else can you explain Transylvania?). As for allowing Serbia, Croatia and Slovenia to go their separate ways - you do realize
that was contradictory to Serbian war aims from the beginning of 1914 - you know, the country that started the war. Across what became Yugoslavia guerilla warfare was fought in support of 'Greater Serbia'. Where those borders lay - or how 'Serbs' could genuinely be identified (and Serbia's government at the time identified Croats as 'Crypto-Serbs' or Serbs that didn't know they were Serbs) - remember they all spoke the same language in effect - was hardly clear. Your scenario would have led to a much great bloodbath probably ending with British and French intervention to lord knows what effect. (But probably with Italy annexing large parts of what's now the Slovenian and Croatian Coast)
Hell, the borders of Bosnia are still in contention
now despite the breaking apart of Yugoslavia into 'coherent' and 'rational' boundaries based on nationality.
Btw, on that note: What nationality would define Macedonians as being? What about in 1911 (when all that region was part of the Ottoman Empire)? What about Albanians - are they one or two peoples; their 'dialects' are often unintelligible to each other...
A solution which would have satisfied no-one - like the post-war treaties. And for someone supporting the breakup of countries, you seem keen on 'Yugoslavia'.
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