speaking two different languages implies not understanding each other, and thus you're implying that a Serbian and a Bosnian can't understand or communicate with each other.

that... is a VERY weird assumption on your end. there is a reason "mutually intelligible languages" is a term, and they don't even have to be as closely related as BCMS. or are you going to argue that danish, norwegian and swedish are the same language? that dutch and german are? italian, spanish, and portuguese?

If your perspective is two different languages can communicate and understand each other perfectly (i.e. Canadian English is one language and american English is another language) than fine. But I don't think anybody cares to have that perspective other than the politicians who manage those technicalities.

correct. i'll do you better: scotland english is also separate from england english, etc. you're literally trying to dismiss established and accepted categorizations because, what, they don't sound right to your "geek interest"? again, there's a reason the concept of mutual intelligibility is a thing.

On the ground, where it is spoken by the people, a person in Pula can speak with a person in niš speaking standard croatian and Serbian. And that's because the formal language is actually stokavian, not Independent Croatian and Serbian etc. the same way the language is English, and not American, Canadian, etc.

yes, now you're getting close to my point: slavic languages are a dialect continuum and any language delineation is inherently materially and politically influenced. to reiterate: BCMS is definitely a thing. but so are standard croatian, standard serbian, standard montenegrin and so on. unless, again, you want to argue that kajkavians are actually speaking slovenian and chakavians speak something else entirely.