[foobarian]

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

[troupo]

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)

[foobarian]

Now that you mention it, there were also several iterations of the original Glagolitic, the angular being my favorite :-)