I do not truly believe South Slavs are Slavic at all since there is a sprachbund that binds us "Slavs" (former Yugoslavia and Bulgaria) and non-Slavs (Hellenes, Albanians and Romanians) together with origins of this such union predating the Slavic invasion of Haemus.
Going by this, there is evidence proving Haemus peoples are much closer than we thought and farther from West/East Slavs. The whole South Slavic thing is really just Russian propaganda for their wet Pan-Slavic dreams i.e. Greater Russia I believe.The term "Balkan linguistic union" was coined by the Romanian linguist Alexandru Rosetti in 1958, when he claimed that the shared features conferred the Balkan languages a special similarity. Theodor Capidan went further, claiming that the structure of Balkan languages could be reduced to a standard language. Many of the earliest reports on this theory were in German, hence the term "Balkansprachbund" is often used as well.