To be fair, the parent post was clearly talking about Slavic languages, not "all the languages USSR forced cyrillic alphabet on", which were not Slavic and which required significant modifications to the alphabet.
Indeed: most notably, Croatian, Slovenian, Bosnian, Serbian and Montenegrin are all unambiguous with Cyrillic, but Latin script dominates, even in officially Cyrillic-first Serbia.
Again, it is seen as a political tool (pro-West or pro-Russia), when Cyrillic is technically better suited (there is certainly history as well, but that's very mixed up in the region).
Again, I am saying this as someone who has worked to implement things like full-text search, collation (lexical ordering/sorting) algorithms and tables, fonts and ligatures, functions like uppercase/titlecase/lowercase...
Eg. an already complex Unicode Collation Algorithm tables can never support exceptions with digraphs like "konjukcija" (nj is usually a digraph, but not here), etc.
The unique quirk with South Slavic languages is the linguistic work e.g. associated with Vuk Karadžić [1] which resulted in a cleaned up purely phonetic alphabet. This was done across the region and ended up getting plumbed through both alphabets, so e.g. the Croatians/Slovenes write in latin but with a handful of special characters for the unique sounds like "š" or the double-letter characters "dž" "lj", which also map 1-1 to stuff on the Serbian Cyrillic side.
It's the kind of legacy cleanup you love to see :-)
I invite you for a walk through Belgrade streets, maybe even with Google Street View. There will be Cyrillic in official signage, but ads and shop names will be predominately in Latin script. If there are some in Cyrillic, they are likely to be part of a newer "hipster" move to differentiate more for the tourists.
> None of those are digraphs or have diacritics, each is a single letter/character
Okay, you got me, these don't have diacritics, but other Slavic languages do. Unicode committee decided that some of these are separate letters (Ѓ, Й, Ё etc.) and some are not (Ў), but still doesn't make these "trivially provable to suit Slavic languages better".
> Nj (this "letter" is made up of two other letters)
Indeed it is. Invented in 1818. E.g. Russian uses two "нь" for the same thing.
My point is, even if I may confuse my linguistic terminology from time to time, is... How does all this make Cyrillic "trivially provable" to be better suited for Slavic languages than Latin script? It's all the same: invent new letters or new letter combinations, or slap a few diacritics on top. And when that is not enough, borrow from Latin. E.g., j in Serbian, ї in Ukrainian and й in Russian for the same sound.
Some of this was a top-down overhaul of the writing system in the 19th century. Before that it was an awful mess and people just "vibe-wrote" the weird Slavic sounds using latin how they saw fit; try reading some old writings from that era. Or read some modern Polish or Czech text :-D
So the original claim of "is trivially provable that Cyrillic script is better adapted even to languages which do not use it today" boils down to "in the 19th Century they cleaned up the writing system" (similarly, there was a top-down cleanup of Russian, with Bolsheviks removing ѣ,ѳ, і, ѵ and ъ version of yer)
Again, it is seen as a political tool (pro-West or pro-Russia), when Cyrillic is technically better suited (there is certainly history as well, but that's very mixed up in the region).
Again, I am saying this as someone who has worked to implement things like full-text search, collation (lexical ordering/sorting) algorithms and tables, fonts and ligatures, functions like uppercase/titlecase/lowercase...
Eg. an already complex Unicode Collation Algorithm tables can never support exceptions with digraphs like "konjukcija" (nj is usually a digraph, but not here), etc.