The existence of a dialect continuum implies that there is no line where a boundary can be drawn.
Exactly. No borders can be drawn by linguistic reasons in a dialect continuum. The borders in such areas have political or religious reasons.
So what if there is a dialect continuum? A language could be closer to a language with which it has no dialect continuum while very far from a language with which it has a dialect continuum, especially if the continuum chain is very long.
If Slovene seems to you closer to Slovak and Czech, this could be because Slovene/Croatian/Czech/Slovak are Viennese Slavic while Bulgarian is a Constantinopolitan Slavic, i.e. there are political reasons.
Тhe Slavic linguistic community developed from a former linguistic community via three major features: 1st palatalization, open syllables, 2nd palatalization.
The
East Slavic group is defined by an
isogloss related to
opening syllables: East Slavic dialects add a syllable in the case of former *-tort- (>*-torot-). The rest of Slavic, the "mainstream" Slavic, use swapping (*-tort- > *-trot-, I ignore the length of the vowel here).
The
West Slavic group is defined by an
isogloss related to the
2nd palatalization: in West Slavic there is CH => SH (Cyrillic notation: X => Ш) while in the mainstream Slavic there is CH => S (Cyrillic notation: X => С).
All outside East and West is South. This is the 1st definition of
South Slavic - mainstream sides of both isoglosses.
Consistency of these definitions: Are there Slavic dialects which are both East and West? This could be a problem, a minor problem, actually.
These definitions use very old language characteristics, before any Viennesity or Constantinopolicy.
According to these definitions, Slovene is South Slavic, Slovak is West Slavic.
Let us go deeper into history.
The Slavophonia developed in the Middle-Danube area (1st palatalization, open syllables, 2nd palatalization).
The Slavophonia was the natural language of the settled population of this part of Europe (Middle Danube).
Before 1000AD, there was one Common Slavic Language with its dialects, as any other spoken language.
The first political division of the Slavophonia occured about 700-800AD.
About 700-800AD, the city of Constantinopol was the center of our world, the political, economic, religious, cultural center. Slavophones migrated toward Constantinopol, to the center of civilization at that time. In this way, the Constantinopolitan Slavophonia (
царское славяногласие) appeared. We could call the rest of Slavophonia, still on the Middle-Danube,
Carolingian (<=click, =>
королевское славяногласие).
Slavo-Balkanic (or
Balkano-Slavic, Bulgarian/Macedonian) has its origins in the
Constantinopolitan Slavophonia. Actually, until the 19-th century, Slavo-Balkanic has ever been Constantinopolitan.
The rest of Slavic, or Mainstream-Slavic, or
Neo-Slavic, has its origins in the
Carolingian Slavophonia.
That first political division of the Slavophonia caused the drastic difference in grammar between Balkano-Slavic and Neo-Slavic.
About 800AD, the Carolingian Slavophonia was brought to Kiev and Novgorod. About a century later, the "Kievese" Slavophonia was brought to Moscow.
Among Constantinopolitan Slavophones, all actually being already Christians and Romans by nationality, there were intellectuals who cared about christening/baptizing of pagan Slavophones. Those translated the Holly Scripture into Slavic (the Constantinopolitan one) thus creating the Old Slavonic Literary language. About 1000AD, that language was brought to Kiev inside the Gospel manuscripts. It later developed into Church-Slavonic.
The question: Should "South Slavic" classification be abolished?
No. South Slavic is the mainstream Slavic according to some very old isoglosses.
Moreover, South Slavic has its own isogloss: soft A-stem noun declension (East and West are mainstream in relation to this isogloss).