> were replaced with "equivalent" Greek or Cyrillic one
The subset of equivalent letters, or different ones? If they looked the same, it wouldn't bother me if the letters in the center were a single codepoint between European languages:
The problem is they don't look the same. So imagine, for instance, Я instead of "R" or И instead of "N" (I don't think the sounds are actually equivalent but let's run with it for the sake of example). Not insurmountable. One could still read a text with these substitutions. But it'd be distracting, and extra detrimental for people who don't speak English as their first language.
The subset of equivalent letters, or different ones? If they looked the same, it wouldn't bother me if the letters in the center were a single codepoint between European languages:
https://upload.wikimedia.org/wikipedia/commons/8/84/Venn_dia...